Final answer:
The U.S. controlled its economic and military decisions post World War I through a selective approach of isolationism and strategic engagement, notably avoiding alliances that required military commitments and using economic means like the Marshall Plan to influence international affairs.
Step-by-step explanation:
Following World War I, the United States demonstrated its intent to control its own economic and military decisions through a return to isolationism. American foreign policy focused on non-intervention, as reflected by the country's decision to refrain from joining the League of Nations, despite President Woodrow Wilson's involvement in its creation. The reluctance to enter into alliances that would require military commitment abroad echoed a historical preference for staying out of European affairs, which was a sentiment shared by many Americans post World War I.
The Marshall Plan represents a pivotal moment where the United States stepped away from complete isolationism by offering economic assistance to foreign nations to combat the spread of Communism, showing a strategic engagement in world affairs. This was a clear example of America acting independently to shape the post-World War II order, without fully committing to foreign military entanglements. However, the overall approach was still cautious and selective, ensuring America's autonomy in decision-making remained intact.
In summary, the U.S. maintained control over its economic and military decisions by employing an isolationist stance after World War I, intervening economically via the Marshall Plan, and by not committing to international military alliances, which would limit its own sovereignty.