Final answer:
The U.S. reinforced its post-WWI isolationist stance through the Neutrality Acts and by refraining from joining the League of Nations, thus avoiding European entanglements.
Step-by-step explanation:
After WWI, the action that showed the U.S.'s effort to isolate itself from European conflicts was the passage of the Neutrality Acts in 1935, 1936, and 1937. These acts prohibited Americans from selling arms, providing loans, or traveling on ships of nations at war, which highlighted the strong isolationist sentiment that had resurfaced in the United States.
Reflecting a policy of avoiding 'entangling alliances' as advised by George Washington and reinforced by the Monroe Doctrine, this isolationist stance was further influenced by a reluctance to repeat the human and economic toll that the country faced due to its involvement in WWI.
The League of Nations played a part in this shift toward isolationism by the U.S. after WWI in that the U.S. chose not to join the League, thereby solidifying its stance of staying clear of European political entanglements and reinforcing its commitment to unilateral action.