Many Democrats supported Bush on Iraq because they mistakenly placed the war in the context of a humanitarian intervention. Yet this guardedly interventionist wing of the party also includes people such as House Democratic leader Nancy Pelosi, who opposed the commitment in Iraq but never stopped believing in the positive uses of American military power. These Democrats are swinging Obama’s way on Syria not for partisan reasons but because he shares their position in the quarrel inside the party.

Nonetheless, Obama faces substantial resistance among Democrats because Vietnam and Iraq turned a large section of the party into principled noninterventionists who set an extremely high bar to any use of America’s armed forces. The same can be said of libertarian Republicans such as Sen. Rand Paul and Rep. Justin Amash. This left-right anti-war coalition has a long American pedigree, going back to the periods before both World War I and World War II.

But that only begins to describe the complexity of the argument on the Republican side. Many in the party instinctively skeptical of foreign entanglements suppressed their doubts during the Bush years. With a Democrat in the White House and 9/11 more than a decade behind us, they now feel they can express them again. This group overlaps with a GOP faction whose one driving ideology involves standing against anything Obama is for.