US hegemony over the past few decades has been based on a combination of military, economic, and technological superiority, an extensive network of alliances, and the ability to exert political influence in various regions of the world. However, the developments of recent years had revealed signs of the gradual erosion of this position, but the American war against Iran accelerated this process to the point where it caused the decline of the hegemonic position of the United States; Because this war, unlike many of America's previous military interventions, took place against an actor who had significant military capabilities, strategic depth, geopolitical position, the capacity to influence energy security and the Strait of Hormuz, resilience against economic pressures, and the capacity for political and social resistance. The main issue of this research is how the American war against Iran has caused the decline of the hegemony of the United States and the transition from a dominant power to a post-hegemonic order. Using Steven Walt's theory of "predatory hegemony" and literature on the cycle and decline of hegemony, the research argues that the United States used material superiority and military power to achieve strategic goals in the war against Iran, but failed to defeat Iran's resistance. The findings of the research show that Iran's resistance and America's failure to achieve the expected strategic goals, while increasing material and political costs, have challenged the credibility of America's deterrence and ability to convert military superiority into political results. From this point of view, the importance of the war is not only summarized in its military results, but it can be seen in its consequences for the credibility of American power, energy security, trust of allies, calculations of rival powers and the structure of power distribution in the international system.