Credibility is a prized word in international politics. Countries that keep their promises and enforce their red lines can be counted on to deter their enemies and assure their allies. Nations, as with human beings, develop reputations, and those that break their pledges are impossible to trust.
As such, the failure of the United States to bomb Syria is bound to empower Iran and reinforce its quest for nuclear arms. The only problem with such assertions is that the historical record suggests that the so-called credibility argument rests on a very thin intellectual rail. America's enemies put premium on its capabilities rather than indications of its resolve. It is Washington's allies, however, that are likely to prove sensitive to its intentions as opposed to its actual power. The events of the past few weeks have probably impacted Israel more so than Iran.
The most tragic application of the credibility argument for the United States was the Vietnam War. During Washington's quarter of century involvement in Southeast Asia, successive administrations never claimed that Vietnam by itself was relevant for U.S. security, but that the failure to stop the advance of communism there was bound to embolden the Soviet Union. America fought in Vietnamese jungles to prove to the Kremlin that once the United States drew its red lines it would use all of its power to achieve its objectives. In one of the ironies of history, the collapse of U.S. efforts in Vietnam was followed a decade and a half later by the demise of the Soviet Union.