A man walks through a destroyed residential area of Saraqib, Syria, on Monday, September 9, following repeated airstrikes by government forces. More than 100,000 people have been killed in Syria since a popular uprising spiraled into a civil war in 2011, according to the United Nations. The Syrian government's suspected use of chemical weapons in a deadly August 21 attack has prompted the United States to urge military action against the regime. Click through to view the most compelling images taken since the start of the conflict. 
A Free Syrian Army fighter stands on a damaged military tank in Zabadani near Damascus on Sunday, September 8. 
A soldier wears a picture of Syrian President Bashar al-Assad on his chest while at a market in old Damascus on September 8. 
A Free Syrian Army fighter prays in a trench in the Damascus suburbs on September 8. 
A Free Syrian Army fighter takes up a shooting position in Aleppo on September 8. 
A Free Syrian Army fighter runs for cover in Raqqa province on September 8. 
Buildings are heavily damaged in the Salah al-Din area of Aleppo on September 8. 
A boy named Issa, 10, carries a mortar shell in a weapons factory of the Free Syrian Army in Aleppo on Saturday, September 7. The boy works with his father in the factory. 
Issa fixes a mortar launcher in the Aleppo weapons factory on September 7. 
A bedroom lies in ruins after clashes between government forces and rebels around Ariha on September 7. 
Rebels prepare explosive devices for fighting with government forces on September 7 in Deir Ezzor. 
Syrian forces are seen in the Syrian Christian town of Maaloula on September 7. 
A Syrian soldier prepares large-caliber bullets for a machine gun in Maaloula on September 7. 
A man stands inside a home damaged by heavy shelling in Arbeen, a suburb of Damascus, Syria, on September 7. 
Men gather on the remains of a destroyed building after reported airstrikes by Syrian government forces in the rebel-held northwestern Syrian province of Idlib on Thursday, September 5. 
Men observe the wreckage of a motorbike in the Syrian province of Idlib on September 5. The Syrian Observatory for Human Rights watchdog said that Syrian war planes bombed rebel-held areas in Idlib, Aleppo, Hama and Lattakia. 
A Free Syrian Army fighter looks through a hole from behind sandbags, while a fellow fighter reads the Quran in Deir Ezzor, Syria, on Tuesday, September 3. The United States and other Western nations blame the Assad regime for a chemical weapons attack that's believed to have killed more than 1,400 people. 
A Free Syrian Army fighter takes cover as he watches forces loyal to President Bashar al-Assad near Hanano barracks in Aleppo on September 3. 
A Free Syrian Army fighter is seen through a hole in a wall of a building in Aleppo on September 3. 
Free Syrian Army fighters walk inside a damaged house in Aleppo on September 3. 
A Free Syrian Army fighter walks through a hole in a wall in Aleppo on September 3. 
Free Syrian Army fighters talk inside a burnt house in Aleppo on September 3. 
A Free Syrian Army fighter peeks through the curtains of a bedroom in Deir ez-Zor, Syria, on Monday, September 2. 
Free Syrian Army fighters take their positions behind piled sandbags, as one of them points his weapon, in Deir ez-Zor on September 2. 
A rebel fighter points his weapon at Syrian regime forces in Deir ez-Zor on September 2. 
Syrian firefighters try to extinguish a fire after a missile hit a residential building in Damascus, Syria, on September 2. 
A U.N. arms expert collects samples during an inspection of a suspected chemical weapons strike site in the Ghouta area outside Damascus on August 29. 
People search for belongings in rubble in Raqqa, Syria, on August 29. 
Free Syrian Army fighters launch a rocket toward forces loyal to Syrian President Bashar al-Assad in Deir Ezzor on August 29. 
A Syrian Kurd uses hay to hide another woman in a training session organized by the Kurdish Women's Defense Units on Wednesday, August 28, in a northern Syrian border village. They're preparing if the area comes under attack. 
Free Syrian Army fighters escort U.N. vehicles with chemical weapons experts on August 28 through a site of a suspected chemical weapons attack outside Damascus. 
A Free Syrian Army fighter takes position behind sandbags in the old city of Aleppo, Syria, on Tuesday, August 27. 
A U.N. team leaves its Damascus, Syria, hotel in a convoy on Monday, August 26. The team was to investigate an alleged chemical attack that killed hundreds last week in a suburb of the Syrian capital. Sniper fire hit a vehicle used by the U.N. chemical weapons investigation team multiple times Monday, according to the United Nations. 
A Syrian soldier walks down a street in Damascus on Saturday, August 24. 
Pigeons lie dead on the ground on August 24 from after what activists say is the use of chemical weapons by government forces in the Damascus suburb of Arbeen. 
Columns of smoke rise in Barzeh after heavy shelling on Friday, August 23. 
A young Free Syrian Army fighter is reflected in a mirror as he takes position in a house in Aleppo on Thursday, August 22. 
Rebels move around a building in Aleppo on August 22. 
Syrian rebels claim pro-government forces used chemical weapons to kill citizens outside Damascus on Wednesday, August 21. People inspect bodies in this photo released by the Syrian opposition Shaam News Network. 
People search the rubble of a bombed building in Aleppo, Syria, on Friday, August 16. 
Men bury the bodies of six members of the same family killed in a bombing in Raqqa on Saturday, August 10. 
Syrian Army soldiers patrol a devastated street in Homs on Wednesday, July 31. 
Free Syrian Army fighters move through a hole in a wall in Khan al-Assal on Monday, July 22, after seizing the town. 
A rebel fighter walks past swings in a deserted playground in Deir al-Zor, Syria, on Sunday, July 21. 
A rebel fighter speaks with a fellow fighter through a hole in a wall in Deir al-Zor on July 21. 
A Free Syrian Army fighter casts a shadow on a wall as he carries his weapon in a shelter in Deir al-Zor on Thursday, July 18. 
Yahya Sweed, 13, is comforted by his father as he lies on a bed in Kfar Nubul on Tuesday, July 16. The boy was injured by shrapnel, resulting in the amputation of his right leg. 
A rebel fighter naps in a trench about 300 feet from the Syrian government forces' positions along the highway connecting Idlib with Latakia on Monday, July 15. 
A rebel fighter uses a hole in the wall of a destroyed school to aim at Syrian government forces in the Izaa district of Aleppo on Sunday, July 14. 
A Free Syrian Army fighter uses a mirror to scope out snipers loyal to Syrian President Bashar al-Assad in Aleppo on Friday, July 12. 
A Free Syrian Army fighter stands over a boy who was injured during shelling in Al-Bara on Monday, July 8. 
Members of the Free Syrian Army fire a homemade rocket toward regime forces in Deir al-Zor on Sunday, June 16. 
Syrian rebels leave their position in the northwestern town of Maaret al-Numan on Thursday, June 13. The White House said that the Syrian government has crossed a "red line" with its use of chemical weapons and announced it would start arming the rebels. 
Forces loyal to Syrian President Bashar al-Assad are seen near Qusayr on Thursday, May 30. 
Syrian rebels take position in a house during clashes with regime forces in the old city of Aleppo on May 22. 
Syrian army soldiers take control of the village of Western Dumayna north of the rebel-held city of Qusayr on Monday, May 13. Syrian troops captured three villages in Homs province, allowing them to cut supply lines to rebels inside Qusayr town, a military officer told AFP. 
Rebel fighters fire at government forces in the northern Syrian city of Aleppo on Sunday, May 12. 
Searchers use a flashlight as they look for survivors among the rubble created by what activists say was a missile attack from the Syrian regime, in Raqqa province, Syria, on April 25. 
A Kurdish fighter from the "Popular Protection Units" (YPG) takes position inside a building in the majority-Kurdish Sheikh Maqsood area of Aleppo, on Apri. 21. 
People walk past destroyed houses in the northern Syrian town of Azaz on Sunday, April 21. 
Free Syrian Army fighters take positions prior to an offensive against government forces in the Khan al-Assal area, near Aleppo on Saturday, April 20. 
Men inspect damage at a house destroyed in an airstrike in Aleppo on April 15. 
Syrian and Kurdish rebel fighters walk in the Sheikh Maqsud district of Aleppo on April 14. 
A female rebel monitors the movement of Syrian government forces in Aleppo's Sheikh Maqsud neighborhood on Thursday, April 11. 
A rebel runs to avoid sniper fire from Syrian government forces in Aleppo on April 11. 
Syrian rebels observe the movement of Syrian government forces around Al-Kendi hospital in Aleppo on Wednesday, April 10. 
Rescue teams and security forces check out the scene of a deadly car bomb explosion in Damascus on April 8. 
The fighting has taken a toll on buildings in Aleppo's Saladin district, seen here on April 8. 
A Syrian rebel runs for cover in Deir ez-Zor on April 2. 
A rebel checks for snipers across the street toward the Citadel in Aleppo, Syria, on Saturday, March 30, in this photo taken by iReporter Lee Harper. 
A Free Syrian fighter mourns the death of a friend in Aleppo on March 30, in this photo taken by iReporter Lee Harper. 
A Syrian opposition fighter runs for cover from Syrian army snipers in Aleppo on Wednesday, March 27. 
A Syrian girl covers her face to protect herself from fumes as a street covered with uncollected garbage is fumigated in Aleppo on Sunday, March 24. 
A Syrian man and his family drive past damaged buildings in Maarat al-Numan, on Wednesday, March 20. 
Syrians carry the body of a Syrian army soldier during a funeral ceremony in Idlib province on Tuesday, March 19. 
Syrian rebels take position in Aleppo, the largest city in the country, on March 11. 
Syrian men search for their relatives amongst the bodies of civilians executed and dumped in the Quweiq River on March 11. 
A Free Syrian Army fighter looks back as smoke rises during fighting between rebel fighters and forces loyal to Syria's President Bashar al-Assad on the outskirts of Aleppo on Saturday, March 2. 
Residents read Shaam News newspapers published by the Free Syrian Army in Aleppo on March 2. 
A member of the Free Syrian Army reacts to the death of a comrade who was killed in fighting, at Bustan al Qasr cemetery in Aleppo on Friday, March 1. 
A rebel fighter throws a home-made grenade at Syrian government forces in Aleppo on February 16. 
A member of the Free Syrian Army stands with his weapon as he looks at a rainbow in Aleppo on February 16. 
A Syrian woman looks through a bus window in Aleppo on February 14. 
Free Syrian Army fighters walk through a dust-filled stairwell in Damascus on February 7. 
A Syrian rebel gestures at comrades from inside a broken armored personnel carrier in Al-Yaqubia on February 6. 
A rebel fighter throws a hand grenade inside a Syrian Army base in Damascus on February 3. 
People stand in the dust of a building destroyed in an airstrike in Aleppo, Syria on February 3. 
Free Syrian Army fighters run as they enter a Syrian Army base during heavy fighting in the Arabeen neighborhood of Damascus on February 3. 
An unexploded mortar shell fired by the Syrian Army sits lodged in the ground in Damascus on January 25. 
Fighters from Fateh al Sham unit of the Free Syrian Army fire on Syrian Army soldiers at a check point in Damascus on January 20. 
A Free Syrian Army fighter walks between buildings damaged during Syrian Air Force strikes in Damascus on January 19. 
A Syrian rebel fighter tries to locate a government jet fighter in Aleppo on January 18. 
Syrian rebels launch a missile near the Abu Baker brigade in Albab on January 16. 
A Syrian boy walks near rubbish next to tents at a refugee camp near the northern city of Azaz on the Syria-Turkey border, on January 8. 
Syrians look for survivors amid the rubble of a building targeted by a missile in Aleppo on January 7. 
A father reacts after hearing of a shelling by forces loyal to Syria's President Bashar al-Assad in Aleppo on January 3. 
A patient smokes a cigarette at Dar Al-Ajaza psychiatric hospital in Aleppo on December 18, 2012. The psychiatric ward, housing around 60 patients, has lacked the means to function properly since fighting broke out there in July. 
Syrians mourn a fallen rebel fighter at a rebel base in the al-Fardos area of Aleppo on December 8, 2012. 
Members of Liwa (Brigade) Salahadin, a Kurdish military unit fighting alongside rebel fighters, monitor the area in the besieged district of Karmel al-Jabl in Aleppo on December 6, 2012. 
A member of Liwa Salahadin aims at a regime fighter in the besieged district of Karmel al-Jabl in Aleppo on December 6, 2012. 
Two young boys sit underneath a washline in a refugee camp on the border between Syria and Turkey near Azaz on December 5, 2012. 
The bodies of three children, who were allegedly killed in a mortar shell attack that landed close to a bakery in Aleppo, on December 2, 2012, are laid out for identification by family members at a makeshift hospital at an undisclosed location of the city. 
Smoke rises in the Hanano and Bustan al-Basha districts in Aleppo on December 1, 2012 as fighting continues through the night. 
Damaged houses in Aleppo are seen after an airstrike on November 29, 2012. 
A Syrian rebel mourns the death of a comrade in Maraat al-Numan on November 20, 2012. 
Syrians protesters stand on Assad's portrait during an anti-regime demonstration in Aleppo on November 16, 2012. 
A Syrian rebel takes cover during fighting against Syrian government forces in Aleppo on November 15, 2012. 
Syrian opposition fighter Bazel Araj, 19, sleeps next to his pistol in Aleppo on November 11, 2012. 
A rebel fighter fires at a Syrian government position in Aleppo on November 6, 2012. 
A Syrian rebel leaps over debris left in the street while running across a "sniper alley" near the Salahudeen district in Aleppo on November 4, 2012. 
Rebels hold their position in the midst of a battle on November 3, 2012 in Aleppo. 
A man cries while being treated in a local hospital in a rebel-controlled area of Aleppo on October 31, 2012. 
A man is treated for wounds after a government jet attacked the Karm al-Aser neighborhood in eastern Aleppo on October 31, 2012. 
A Syrian rebel interrogates a handcuffed and blindfolded man suspected of being a pro-regime militiaman in Aleppo on October 26, 2012. 
Smoke rises from a fuel station following a mortar attack as Syrian women walk on a rainy day in the Arqub neighborhood of Aleppo on October 25, 2012. 
A Syrian rebel fires at an army position in the Karm al-Jabal district of Aleppo on October 22, 2012. 
A wounded Syrian boy sits on the back of a truck carrying victims and wounded people to a hospital following an attack by regime forces in Aleppo on October 21, 2012. 
A man lies on the ground after being shot by a sniper for a second time as he waits to be rescued by members of the Al-Baraa Bin Malek Battalion, part of the Free Syria Army's Al-Fatah brigade, in Aleppo on October 20, 2012. 
Syrian army soldiers run for cover during clashes with rebel fighters at Karam al-Jabal neighborhood of Aleppo on October 20, 2012. 
Smoke rises after a Syrian Air Force fighter jet fired missiles at the suburbs of the northern province of Idlib on October 16, 2012. 
A Syrian opposition fighter stands near a post in Aleppo on October 11, 2012. 
A Syrian man mourns the death of his father, who was killed during a government attack in Aleppo on October 10, 2012. 
A rebel fighter is carried by his friends and laid on a gurney to be treated for gunshot wounds sustained during heavy battles with government forces in Aleppo on October 1, 2012. 
Syrian rebels help a wounded comrade to an Aleppo hospital after he was injured in a Syrian army strike on September 18, 2012. 
Free Syria Army fighters are reflected in a mirror they use to see a Syrian Army post only 50 meters away in Aleppo on September 16, 2012. 
A Syrian man carrying grocery bags tries to dodge sniper fire as he runs through an alley near a checkpoint manned by the Free Syria Army in Aleppo on September 14, 2012. 
A woman walks past a destroyed building in Aleppo on September 13, 2012. 
Free Syrian Army fighters battle during street fighting against Syrian army soldiers in Aleppo on September 8, 2012. 
A Syrian man wounded by shelling sits on a chair outside a closed shop in Aleppo on September 4, 2012. 
A woman sits in her wheelchair next to her house, damaged by a Syrian air raid, near Homs on August 26, 2012. 
Members of the Free Syrian Army clash with Syrian army soliders in Aleppo's Saif al-Dawla district on August 22, 2012. 
A man mourns in front of a field hospital on August 21, 2012 in Aleppo. 
Wounded civilians wait in a field hospital after an air strike on August 21, 2012 in Aleppo. 
People pray during the funeral of a Free Syrian Army fighter, Amar Ali Amero, on August 21, 2012. 
A man cries near the graves of his two children killed during a recent Syrian airstrike in Azaz on August 20, 2012. 
A Syrian woman holds her dead baby as she screams upon seeing her husband's body being covered following an airstrike by regime forces on the town of Azaz on August 15, 2012. 
A Syrian rebel runs in a street of Selehattin during an attack on the municipal building on July 23, 2012. 
Syrian rebels hunt for snipers after attacking the municipality building in the city center of Selehattin on July 23, 2012. 
Members of the Free Syrian Army's Mugaweer (commandos) Brigade pay their respects in a cemetery on May 12, 2012 in Qusayr. 
Syrian rebels take position near Qusayr on May 10, 2012. 
A Free Syrian Army member takes cover in underground caves in Sarmin on April 9, 2012. 
Rebels prepare to engage government tanks that advanced into Saraquib on April 9, 2012. 
Men say prayers during a ceremony in Binnish on April 9, 2012. 
A young boy plays with a toy gun in Binnish on April 9, 2012. 
A Free Syrian Army rebel mounts his horse in the Al-Shatouria village near the Turkish border in northwestern Syria on March 16, 2012, a year after the uprising began. 
Syrian refugees walk across a field before crossing into Turkey on March 14, 2012. 
A rebel takes position in Al-Qsair on January 27, 2012. 
A protester in Homs throws a tear gas bomb back towards security forces, on December 27, 2011. 
A man stands under a giant Syrian flag outside the Umayyad Mosque in Damascus on December 24, 2011. 
A member of the Free Syrian Army looks out over a valley in the village of Ain al-Baida on December 15, 2011. 
Members of the Free Syrian Army stand in an valley near the village of Ain al-Baida, close to the Turkish border, on December 15, 2011. 
Displaced Syrian refugees walk through an orchard adjacent to Syria's northern border with Turkey on June 14, 2011, near Khirbet al-Jouz. 
A Syrian man holds up a portrait of President Bashar al-Assad during a rally to show support for the president in Damascus on April 30, 2011. 
Syrians rally to show their support for President Bashar al-Assad in Damascus on April 30, 2011. 
A screen grab from YouTube shows thick smoke rising above as Syrian anti-government protesters demonstrate in Moaret Al-Noman on April 29, 2011. 
A screen grab from YouTube shows Syrian anti-government protesters run for cover from tear gas fired by security forces in Damascus on April 29, 2011, during the "Day of Rage" demonstrations called by activists to put pressure on al-Assad. 
A woman sits by the hospital bed of a man allegedly injured when an armed group seized rooftops in Latakia on March 27, 2011, and opened fire at passers-by, citizens and security forces personnel according to official sources. 
Syrians wave their national flag and hold portraits of al-Assad during a rally to show their support for their leader in Damascus on March 29, 2011. 
Syrian protesters chant slogans in support of al-Assad during a rally in Damascus on March 25, 2011. 
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- U.S. intelligence agencies had some prior warnings about Syria's chemical attack
- Schneier: Despite the knowledge, the U.S. wasn't able to stop the attack
- He says having more data does not necessarily enable government to act meaningfully
- Schneier: The paradox is that a lot of valuable intelligence can't be used for action
Editor's note: Bruce Schneier is a security technologist and author of "Liars and Outliers: Enabling the Trust Society Needs to Survive."
(CNN) -- We recently learned that U.S. intelligence agencies had at least three days' warning that Syrian President Bashar al-Assad was preparing to launch a chemical attack on his own people, but wasn't able to stop it. At least that's what an intelligence briefing from the White House reveals. With the combined abilities of our national intelligence apparatus -- the CIA, National Security Agency, National Reconnaissance Office and all the rest -- it's not surprising that we had advance notice. It's not known whether the U.S. shared what it knew.
More interestingly, the U.S. government did not choose to act on that knowledge (for example, launch a pre-emptive strike), which left some wondering why.
There are several possible explanations, all of which point to a fundamental problem with intelligence information and our national intelligence apparatuses.
The first possibility is that we may have had the data, but didn't fully understand what it meant. This is the proverbial connect-the-dots problem. As we've learned again and again, connecting the dots is hard. Our intelligence services collect billions of individual pieces of data every day. After the fact, it's easy to walk backward through the data and notice all the individual pieces that point to what actually happened. Before the fact, though, it's much more difficult. The overwhelming majority of those bits of data point in random directions, or nowhere at all. Almost all the dots don't connect to anything.
Rather than thinking of intelligence as a connect-the-dots picture, think of it as a million unnumbered pictures superimposed on top of each other. Which picture is the relevant one? We have no idea. Turning that data into actual information is an extraordinarily difficult problem, and one that the vast scope of our data-gathering programs makes even more difficult.
The second possible explanation is that while we had some information about al-Assad's plans, we didn't have enough confirmation to act on that information. This is probably the most likely explanation. We can't act on inklings, hunches, or possibilities. We probably can't even act on probabilities; we have to be sure. But when it comes to intelligence, it's hard to be sure. There could always be something else going on -- something we're not able to eavesdrop on, spy on, or see from our satellites. Again, our knowledge is most obvious after the fact.
The third is that while we were sure of our information, we couldn't act because that would reveal "sources and methods." This is probably the most frustrating explanation. Imagine we are able to eavesdrop on al-Assad's most private conversations with his generals and aides, and are absolutely sure of his plans. If we act on them, we reveal that we are eavesdropping. As a result, he's likely to change how he communicates, costing us our ability to eavesdrop. It might sound perverse, but often the fact that we are able to successfully spy on someone is a bigger secret than the information we learn from that spying.
This dynamic was vitally important during World War II. During the war, the British were able to break the German Enigma encryption machine and eavesdrop on German military communications. But while the Allies knew a lot, they would only act on information they learned when there was another plausible way they could have learned it. They even occasionally manufactured plausible explanations. It was just too risky to tip the Germans off that their encryption machines' code had been broken.
Syria chemical attack: what do we know? 
Obama releases intelligence assessment The fourth possibility is that there was nothing useful we could have done. And it is hard to imagine how we could have prevented the use of chemical weapons in Syria. We couldn't have launched a pre-emptive strike, and it's probable that it wouldn't have been effective. The only feasible action would be to alert the opposition -- and that, too, might not have accomplished anything. Or perhaps there wasn't sufficient agreement for any one course of action -- so, by default, nothing was done.
All of these explanations point out the limitations of intelligence. The NSA serves as an example. The agency measures its success by amount of data collected, not by information synthesized or knowledge gained. But it's knowledge that matters.
The NSA's belief that more data is always good, and that it's worth doing anything in order to collect it, is wrong. There are diminishing returns, and the NSA almost certainly passed that point long ago. But the idea of trade-offs does not seem to be part of its thinking.
The NSA missed the Boston Marathon bombers, even though the suspects left a really sloppy Internet trail and the older brother was on the terrorist watch list. With all the NSA is doing eavesdropping on the world, you would think the least it could manage would be keeping track of people on the terrorist watch list. Apparently not.
I don't know how the CIA measures its success, but it failed to predict the end of the Cold War.
More data does not necessarily mean better information. It's much easier to look backward than to predict. Information does not necessarily enable the government to act. Even when we know something, protecting the methods of collection can be more valuable than the possibility of taking action based on gathered information. But there's not a lot of value to intelligence that can't be used for action. These are the paradoxes of intelligence, and it's time we started remembering them.
Of course, we need organizations like the CIA, the NSA, the NRO and all the rest. Intelligence is a vital component of national security, and can be invaluable in both wartime and peacetime. But it is just one security tool among many, and there are significant costs and limitations.
We've just learned from the recently leaked "black budget" that we're spending $52 billion annually on national intelligence. We need to take a serious look at what kind of value we're getting for our money, and whether it's worth it.
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The opinions expressed in this commentary are solely those of Bruce Schneier.
