By synthesizing material forces with ideational forces more organically via a social evolutionary approach, we advance a deeper understanding about post-World War II American military interventionism. We argue that post-World War II American military interventionism — that is, the American elites’ and public’s support for America’s military intervention abroad — cannot be understood with ideational or psychological forces alone. Rather, two crucial material variables, namely, geography and aggregate power amplified by superior technological prowess, are indispensable for understanding the propensity for the United States to intervene militarily abroad. These two factors have powerfully shielded the American elites and public from the horrendous devastation of war. As a result, compared to their counterparts in other major states, American citizens and elites have tended to be less repelled by the prospect of war. The outcome is that since World War II the United States has been far more active in military intervention overseas than other major states.