Benjamin Friedman does  a fine job of dismantling the “credibility” argument:
Historical studies show that leaders deciding whether to defy foreign threats focus on the balance of military power and the material interests of the threatening state, not on its opponent’s record of carrying out past threats. Credibility doesn’t travel well.
That is why the domino theory was wrong. Neither the West Europeans we were defending during the Cold War, nor their Warsaw Pact adversaries believed that U.S. withdrawal in Vietnam would mean U.S. abandonment of Europe’s defense. The same goes for other U.S. military actions in the last several decades that ended badly—for example, the Marine deployment to Lebanon under President Reagan and the recent war in Iraq. Presidents initially offered big talk about goals. Later, we quit without having reached those goals. Contrary to claims of credibility hawks, other U.S. allies did not lose faith in American military power or come under attack from emboldened foes. Instead, new supplicants continued to ask for our help [bold mine-DL]. Often, when it was not forthcoming, or too limited for U.S. hawks, they insisted that we would lose credibility if we did not do more. They always proved wrong.
The strange thing is that “credibility” hawks’ warnings continue to be taken seriously when, as Friedman says, they haven’t ever been right. The fact that they’ve never been right should tell us that there is something inherently wrong with the concept they keep using. Considering how many times U.S. “credibility” has supposedly been shattered or ruined, it is remarkable how many dozens of eager would-be clients and long-standing allies still line up with Washington and fully expect the U.S. to protect them and/or do as they wish. Warning about “credibility” is a giveaway that the person issuing the warning has run out of persuasive arguments and has nothing else left. Friedman sums it up this way:
A good rule of thumb for foreign policy is that if someone tells you our credibility depends doing something, it’s probably a bad idea.
This true not only because “credibility” hawks are always invoking credibility in order to justify more aggressive policies in places of little or no importance to the U.S., but because the reliance on the “credibility” argument is confirmation that these policies can’t be defended on the merits. The arguments for deeper U.S. involvement in conflicts that are at best tangentially related to U.S. vital interests are not compelling ones, which is why the “credibility” argument is used so often in these debates. “You may not agree with doing X, but you don’t want to risk encouraging a North Korean invasion, do you?” At its core, the “credibility” argument is a sort of extortion: if you don’t agree to do what the hawks prefer in one place, your actual allies somewhere else are supposedly going to get hurt. This should alert us to the weakness of the policy arguments, but instead many Americans allow themselves to be tricked into letting “credibility” concerns overrule all of their objections.
“Credibility” hawks also have the bad habit of exaggerating the significance of the commitment that the U.S. made in the past to pretend that the supposed “credibility” gap is far greater than it is. Consider the infamous “red line” over Syrian chemical weapons. It’s true that Obama shouldn’t have drawn this line, but he did so in such a vague, almost meaningless way that he had not really committed the U.S. to any particular course of action. It was Syria hawks that latched onto the “red line” and declared that it was a promise to intervene militarily. Similarly, the U.S. had made no commitments to defend Ukraine, but by pretending that the U.S. was ignoring its commitments in the Budapest memorandum “credibility” hawks insisted on taking a harder line in the crisis so that real American security commitments elsewhere wouldn’t be undermined. That’s how it often works: the “credibility” hawks insist on adding major new commitments that the U.S. never even contemplated having before, and then declare the entire alliance system and security of the planet at risk unless the rest of us agree with whatever reckless and unnecessary scheme they have devised. The “credibility” argument is nothing more than a scam, and we are the marks.