Final answer:
Before 1941, the United States was involved in one major European conflict, World War I, but afterward returned to a policy of isolationism until entering World War II in 1941 due to the Pearl Harbor attack.
Step-by-step explanation:
Before 1941, the United States had gotten involved with one major war in Europe, which was World War I. The U.S. formally entered into World War I in 1917, three years after the war began in Europe. Despite the intention for World War I to be the 'war to end all wars' as hoped by President Woodrow Wilson, the interwar period saw the United States returning to a policy of isolationism, focusing on domestic issues during the Great Depression.
During this time, the U.S. avoided further involvement in European conflicts, even as totalitarian regimes began to rise in Europe. It was not until the attack on Pearl Harbor on December 7, 1941, by Japan that the U.S. was compelled to enter World War II.