The scale of the American Empire—its move from (mostly) hemispheric to global domination—only began to change in the 1930s. In fact, in my opinion the entire way Americans understand and frame geopolitics was forged during this decade, perhaps the most influential in modern US history.
The primary impetus behind the transformation in the American imperial imagination was the rise of Nazi Germany. Before Adolf Hitler’s ascendance to the German chancellorship in 1933, Americans generally believed they could negotiate with foreign powers—especially white, European powers—in good faith. But the rise of Hitler in Germany (and Stalin in the Soviet Union, though he was less important) persuaded Americans that there were some people with whom one just couldn’t reason—or, as a well-known book from the early 1940s put it, You Can’t Do Business with Hitler.
Hitler thus decisively ended two American dreams: first, that reason could replace violence in international relations (or at least, international relations outside the Western Hemisphere); and second, that the United States could afford to remain aloof from European affairs. In fact, many of the first generation of “defense intellectuals” who would staff and build the US national security establishment during and after World War II were social democrats who in the 1930s embraced a pessimistic theory of geopolitics because they believed Hitler posed a uniquely existential threat to “Western civilization.”
Put another way, it was in the 1930s that many American elites endorsed what scholars refer to as a “Schmittian” understanding of geopolitics. Now, it’s crucial to understand who Carl Schmitt was because he was one of the most influential thinkers of the twentieth century. In brief, Schmitt was a German legal theorist (he eventually became a Nazi) who in his famous The Concept of the Political (1932) argued that “the specific political distinction to which political actions and motives can be reduced is that between friend and enemy.” In simpler terms, Schmitt claimed that politics needed to be understood as a struggle between those who were on your side (friends) and those who were not (enemies). To Americans in the 1930s, Hitler and Nazi Germany as a whole were Schmittian enemies with whom one could not negotiate.
Needless to say, Americans were right to view Hitler and the Nazis as existential enemies. They were vicious brutes who did really want to conquer and dominate a significant portion of the world. The problem, however, was that too many Americans concluded that all geopolitics after Hitler were Schmittian geopolitics. That is to say, Americans began to argue that it was an ontological fact that international relations was a Manichean sphere in which there were good guys (Americans) and bad guys (anyone who disagreed with or challenged Americans). Indeed, the very terms “good guys” and “bad guys” only took off after World War II. In the late-1940s and beyond, a simple moralism permeated American politics and culture.