In this paper I question the morality of U.S. military supremacy or hegemony in terms of what constitute the legitimate use of military force and the proper preparation for using such force. I first discuss in a somewhat synoptic fashion how American hegemonic military force (from its very beginning with the dropping of the atomic bomb on Hiroshima) has been justified in dishonest ways and wrongly executed. Next, I show that Just War Theory (JWT) needs to be revised in order to come to a convincing assessment of U.S. military hegemony and its use of military force. This leads me to propose “just military preparedness,” consisting of five principles of just military preparedness, as a new category of JWT. The failure of the United States to satisfy the principles puts into question its very capability of justly resorting to military force, of lawfully executing force, and of establishing a just peace after war. The principles also point to a more humane alternative of how the United States could meet security threats and sustain a peaceful international order.