I've been thinking a lot about citing credibility to use force, and it's very troubling because for the most part it is imaginary. Instead, it invokes some sense of damaged manliness. HULK MUST SMASH OR LOOK LIKE WEENIE!
Now, before I go any further, let me say to those who invoke the memory of Vietnam, there is no thought of sending American combat troops to Central America.
If we cannot defend ourselves we cannot expect to prevail elsewhere. Our credibility would collapse, our alliances would crumble, and the safety of our homeland would be put in jeopardy.
--President Ronald Reagan, April 27, 1983. From Ronald Reagan, Speaking My Mind, pp. 155 and 160.
Does that sound familiar? I encourage you to go find that speech, which is filled with more inaccuracies and falsehoods than I could count. It's true that we only used "advisors" but we sure helped kill a lot of people.
And yes, I know the contexts are different. Obviously. But the credibility argument is terribly deceptive and dangerous.