A Free Syria Army fighter watches from his position during a fight in Aleppo, 2012. (Ahmed Jadallah / Courtesy Reuters)

In late April, U.S. intelligence services concluded that sarin gas -- a chemical weapon estimated to be 500 times more powerful than cyanide -- had been used in Syria’s ongoing civil war. The White House, which had previously hinted that the Assad regime’s use of chemical weapons would be a trigger for intervention, has been quick to emphasize that it is not yet sure whether that trigger has been pulled. “I’ve got to make sure I’ve got the facts,” U.S. President Barack Obama said in a speech at the end of the month, adding that the United States needs clearer evidence of how the chemical weapons were used and who exactly used them.

That is a tall order. Gathering hard evidence in such a closed country is an inherently difficult task; those who think otherwise should remember the faulty intelligence estimates indicating that Saddam Hussein had amassed extensive stores of weapons of mass destruction in Iraq. That misinformation launched a costly, tremendously controversial, and internationally divisive war -- a cautionary tale for those now responsible for compiling intelligence in Syria.

If the Obama administration is serious in its quest to accumulate corroborating facts about Syrian President Bashar al-Assad’s use of chemical weapons, it should start from an understanding of what will not happen. There will be no full-fledged, transparent, and open investigation with the cooperation of the Syrian government. That seems like it would go without saying, yet in a letter to Congress, the White House expressed its preference to have a “comprehensive United Nations investigation” on the matter. Not only has Assad refused to allow UN inspectors into the country, his government’s own messaging has been consistently opaque, even contradictory: On April 25, the Syrian information minister announced that his country does not have such weapons, only to add that the army would not use them, “assuming the existence of such a weapon in the first place.”

Ultimately, intelligence gathering in Syria will be a piecemeal process. Washington will have to combine different kinds of evidence from its various intelligence bodies (including the Central Intelligence Agency, Defense Intelligence Agency, National Security Agency, and others) with the intelligence agencies of its allies active in the region -- most notably, Britain, France, and Israel.

Signals intelligence -- that is, intercepted communications and satellite imagery -- will inevitably play a big role. Intelligence agencies should focus on intercepting radio and digital communications between military units suspected of handling the chemical weapons; if those units are aware of the movement or deployment of chemical arms, it is likely that they will eventually discuss the matter. These agencies should also sift through satellite images to see whether there is any visual evidence of the movement of chemical weapons. Yet even this would fall well short of the legally watertight evidence that the Obama administration has suggested it is looking for.

That is why human intelligence -- that is, intelligence gathered through direct contact with people -- will be pivotal as well. Of course, the highest order human intelligence will be difficult to collect; infiltrating a foreign organization (such as the Syrian government or military) on short notice is nearly impossible. But infiltration is not the only way to get things done. Agents already on the ground, which the United States is reported to have, could procure samples of human tissue from chemical weapons victims, or soil samples that might verify the use of chemical weapons. (Indeed, it has been reported that British intelligence services have already secured such evidence.) Those same agents could also collect testimony from doctors and medical staff who may have treated casualties of chemical attacks.

If the Assad regime begins to totter, U.S. intelligence agents should be ready to convince sources within the Syrian government to offer information as a way of buying insurance for their own survival. For now, it will likely be easiest to convince Syrian rebels to provide details. But that is also problematic. The opposition could easily manipulate the evidence to encourage outside intervention, as was in part the case in the lead-up to the Iraq war, when a stream of defectors from Iraq provided alarming reports to Western intelligence agencies about Saddam’s capabilities.

There are historical examples of successful intelligence gathering under severe constraints such as these. Take, for instance, the Allies’ struggle to enforce disarmament on a defeated Germany after World War I. The German army was to be reduced to 100,000 men, and the country was banned from having an air force, heavy guns, and submarines. Germany, of course, had every interest in not following the rules. Allied inspectors faced obfuscation at every turn and even physical assault. They were also blind to Germany’s secret cooperation with the Soviet Union on airplane and tank training and on the testing of poison gas.

Even so, the inspectors scored several triumphs: With the help of informants, they managed to identify numerous weapons caches hidden in Germany despite encountering resistance from the German military and government.

The overall mission, in spite of its frustrations, was remarkably successful, as the inspectors managed to strip Germany of most heavy armaments and impede its military recovery as an offensive force for quite some time. Indeed, the biggest lesson from the postwar German case is also a remarkably simple one: Even in the most extraordinary circumstances, persistence can yield important intelligence results.

There is one other lesson as well: After intelligence is collected, the really crucial question is how it will be used. As John Keegan, the eminent military historian, concluded in his Intelligence in War, “There is no such thing as the golden secret, the piece of ‘pure intelligence,’ which will resolve all doubt and guide” a leader to the singular correct course of action. Intelligence only informs policy, which itself is a function of will and calculation. After World War I, the weapons inspectors’ frustrations and the illegal arms discovered were initially used to push a harder line against Germany. Similarly, in the case of Iraq, policymakers decided to place an enormous emphasis on intelligence findings about weapons of mass destruction in making the case for war -- a decision that eventually had a devastating impact on public opinion about the conflict when those weapons were nowhere to be found.

With Syria, it seems a different story is developing, with the Obama administration preferring to use intelligence collection as a way to delay making a clear policy choice. It seems that U.S. officials know that when they call for an international consensus on the facts in Syria, they are asking for something that will be essentially impossible to achieve.