Here's why the U.S. is so sure that Syrian president Bashar al-Assad's government was behind a deadly chemical attack on August 21: U.S. intelligence listened in on a phone call between a Syrian Ministry of Defense official and someone at the country's chemical defense unit. That call, according to a report at Foreign Policy, is more or less why the U.S. is certain that Assad's government bears responsibility for the massacre near Damascus.

Based on the evidence, the U.S. is all but certain to take some course of limited military retaliation against the Syrian army for the chemical attacks carried out in Syria. While President Obama is still officially considering his options, there's little evidence to suggest that the U.S. will continue to stick solely to a diplomatic approach towards the country. 

Previous reports have indicated that the U.S. was relying in part on intercepted communications relating to the chemical attack in their assessment. According to CNN, an intelligence report on Syria, planned for public release in the coming days, will include intercepted intelligence along with forensic information. That report is being compiled by the Office of the Director of National Intelligence, and could be released by Thursday. Rumors of an intercepted phone call were picked up widely by the conservative press for days after the German magazine Focus spoke to an anonymous Israeli intelligence official, who claims that the intercepted phone call comes from intelligence gathered by one of their elite units. Foreign Policy, however, specifies that U.S. intelligence overheard the call.  

But as Foreign Policy's report explains, the call's content doesn't answer many questions still out there about the Assad regime's culpability here: 

 Was the attack on August 21 the work of a Syrian officer overstepping his bounds? Or was the strike explicitly directed by senior members of the Assad regime? "It's unclear where control lies," one U.S. intelligence official told The Cable. "Is there just some sort of general blessing to use these things? Or are there explicit orders for each attack?" 

Nor are U.S. analysts sure of the Syrian military's rationale for launching the strike -- if it had a rationale at all. ... "We don't know exactly why it happened," the intelligence official added. "We just know it was pretty fucking stupid."

The New York Times also picked up on the "why" angle of the Syrian chemical attack on Tuesday evening. 

But the U.S. hasn't promised to answer every question about the chemical attack before announcing a response: as Secretary of State John Kerry laid out on Monday, the burden of proof as far as the government is concerned seems to be directed toward whether the attack happened, and whether Assad's government is somehow responsible. Now that the U.S. believes their case is strong enough on both parts, their response quickly approaches. (Update, 11:10 p.m.: CNN's Frances Townsend reports that the National Security Council is meeting tonight on Syria) 

Meanwhile, there's a less-clear outlook for the effectiveness of any military retaliation against Syria, which everyone is assuming at this point will be along the lines of a "surgical" air strike. Among those with doubts? The leaders of the opposition's Free Syrian Army, who point out that the group hasn't even received the weapons Obama promised to send their way months ago. Opposition leaders also worry that anything less than a major military show of force from the U.S. could leave the Syrian regime in a great position to gain more support within the country, according to the Wall Street Journal