The foundational assumption — certainly the subtext — of many arguments for hitting Assad is that America's reputation is on the line. It's said that many bad things will happen if Obama folds: Various friends and allies will doubt America's pledges to protect them; adversaries (Iran, North Korea, Hezbollah, Al Qaeda and others), smelling weakness, will act with impunity.

"Credibility" has great power in national security debates. It conveys strategic sagacity by using historical analogies. (Neville Chamberlain at Munich is a staple.) It warns of consequences that transcend specific nations or issues. It points to the "big picture" and to complex interconnections. It invokes the United States' unique responsibilities for maintaining global order.

In reality, the credibility gambit often combines sleight of hand with lazy thinking (historical parallels tend to be asserted, not demonstrated) and is a variation on the discredited domino theory. This becomes apparent if one examines how it is being deployed in the debate on Syria.

Obama made a bad decision by publicly, and needlessly, warning a brutal strongman that the United States would resort to military force were he to use chemical weapons. With the White House having announced that Assad had done just that, Obama appears tangled in his own red lines.

But he should not make another mistake now just because he made one earlier. Yet that's what those who invoke credibility in effect recommend because they don't explain convincingly why it's important for him to prove his resolve in this instance. They present credibility as an end in itself, not as a means to achieve a desired outcome, which is what it is.