Stephen Walt takes  a turn at demolishing the awful “credibility” argument:
Unfortunately, this obsession with credibility was misplaced. For one thing, a state’s “reputation” for being tough or reliable didn’t work the way most foreign-policy elites thought it did. American leaders kept worrying that other states would question the United States’ resolve and capability if it ever abandoned an unimportant ally, or lost some minor scrap in the developing world. But as careful research by Ted Hopf, Jonathan Mercer, and Daryl Press has shown, states do not judge the credibility of commitments in one place by looking at how a country acted somewhere far away, especially when the two situations are quite different.
I would just add a few comments to supplement what Walt says about this. The “credibility” argument is almost exclusively used by foreign policy hawks, and they pay no attention to negative international reactions to U.S. behavior that contradict their assumptions about “credibility.” If other states react to provocative and confrontational policies by becoming more assertive in their respective regions, hawks interpret that as proof of the other states’ inherent aggressiveness and “expansionist” tendencies.
Hawks usually don’t accept that adverse responses that directly follow U.S. actions have any connection to U.S. policies, but any development that happens to take place after the U.S. “fails” to “act” somewhere is preposterously traced back to the moment of “inaction.” Thus the U.S. is blamed for somehow “causing” unrelated events in one part of the world by choosing not to do something in an entirely different part, but it is excused from responsibility for the direct negative consequences of whatever it has actually done. That’s because the only thing that jeopardizes “credibility” in their eyes is “inaction” (i.e., not attacking or threatening to attack someone), and adverse consequences of “action” (e.g., expanding alliances, invading/bombing/occupying other countries) are ignored or spun as the result of later “weakness.” The appeal to “credibility” is the bludgeon that hawks employ when the case for their preferred “action” is extremely weak, which is quite often. That bludgeon would have much less effect if more people recognized that the “credibility” argument is entirely without merit.