This downward spiral, the third phase of attitudes about intervention, shapes the Syria debate. The past decade’s reversals have culled the liberal-hawk strain not only among American Democrats but also in the U.K, where Blair’s Labor Party opposed Syrian intervention en masse. Among American conservatives, GOP Sens. John McCain and Lindsey Graham wave the Bush-era banner when they demand an expanded campaign to topple Syria’s Bashar al-Assad. But former House Speaker Newt Gingrich came closer to the Right’s new consensus when he declared recently, “It may be that our capacity to export democracy is a lot more limited than we thought.”

The irony is that Obama plainly shares Gingrich’s skepticism: He is proposing constrained military action against Syria not to overthrow Assad or even primarily to protect civilians. Instead, Obama is narrowly defending the international norm against the use of chemical weapons, and he is seeking to discourage other rogue nations from employing them. Obama’s doubts about Washington’s ability to spread freedom through force more closely track the elder Bush’s Greatest Generation caution than the missionary baby-boomer optimism of Clinton, Blair, and Bush II.

Steinberg, now dean of Syracuse University’s Maxwell School, argues Syria is an especially tough case politically because specific frustration with the Middle East compounds the broader cooling toward humanitarian intervention. Humanitarian arguments, he adds, will likely persuade Congress less than the “realist case” that sparing Assad would embolden rogue states like Iran. Even if Congress backs Obama, this debate’s real lesson is the U.S. has come full circle—and is again focused more on the risks than the rewards of remaking complex societies at gunpoint.