Americans recoiled from Obama’s proposal to attack Syria not only because they are skeptical about military adventures in general but because they weren’t convinced that this particular venture was in the national interest.

“This was kind of a worst case,” said Andrew Kohut, the Pew Center’s founding director. “The public is very gun-shy about intervention, but especially in the Middle East, and especially in a case where the direct U.S. interest isn’t clear. If there were a direct and major threat to the United States, you’d probably see a different picture.”

Indeed, polls taken before earlier conflicts have shown that most Americans are willing to support military action when they are convinced that U.S. security is directly threatened — as they did, for example, when they were convinced (wrongly) by President George W. Bush that Iraq’s Saddam Hussein was building nuclear weapons. On the flip side, most Americans will not support military intervention for purely humanitarian reasons — as Bill Clinton learned in Somalia, Bosnia and Kosovo, operations that were all widely unpopular at the time.

That’s a problem Obama hasn’t solved when it comes to Syria. He asked Americans to watch videotapes of children choking on sarin gas in a Damascus suburb — but that was a humanitarian appeal, not an invocation of national security. He argued that Americans had an interest in bolstering international norms against chemical weapons — but that sounded like an abstract principle, not an immediate threat.