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Stephen Walt considers the reasons why many realists have usually opposed U.S. involvement in foreign conflicts. Here he addresses the “credibility” issue:
Realists are also sanguine about U.S. credibility, and do not believe America has to fight wars in places that don’t matter in order to convince its allies and adversaries that it will fight in places that do. Indeed, realists understood that wasting resources on pointless wars might actually undermine your credibility, especially if it left the nation weaker or war-weary. Staying out of a quagmire like Syria and declining to intervene in Crimea tells the world precisely nothing about whether the U.S. commitment to defend its NATO or Asian allies or its other genuine interests; indeed, our other commitments will be easier to meet if we aren’t distracted by peripheral conflicts of little strategic importance [bold mine-DL].
Adding to this, I would just say that both realists and non-interventionists are also much more likely to remember that U.S. resources and power are limited and they need to be husbanded rather than constantly expended. If the U.S. were guided by the hawkish obsession with “credibility,” it would mean that the U.S. would be constantly overstretched and forced to participate in conflicts that it could have easily avoided without danger. The hawks’ “credibility” arguments are wrong, but to act as if they were true would require the U.S. to have virtually inexhaustible resources–including political support at home–that it very clearly doesn’t have available. The impulse to demonstrate “strength” and supposedly preserve “credibility” is what leads the U.S. to fritter away its resources on unnecessary conflicts, and puts our country in a worse position to protect those things that truly are of vital importance.