The U.S. response to war in Europe had evolved greatly over time. Since our foreign policy changes as the strength of our own nation grew, our response has never been uniform.
During the presidency of George Washington, we had no navy and only a small army. Washington new that we were in no shape militarily to tangle with powerful European nations, so he established a policy of neutrality, where we took no part in European wars or alliances.
By the beginning of the 1900’s, our nations military and economic power had grown substantially, so our foreign policy shifted. We were more involved in world politics since we had economic and territorial interest abroad. We were also more respected as a nation thanks to our victory in the Spanish American War, where we defeated Spain and took over many of its former colonies.
However, when World War I broke out, we remain neutral, despite some people wishes to side with the British. Although we did loan money to the Allies and some Americans volunteered thought the Foreign Service, we generally stayed out of the war because we felt it didn’t directly affect us. It was only after Germany sunk a ship carrying U.S. passengers and the amount of debt owed to us by the Allies grew overly large did we decide to enter the war and help end it.
World War II was similar in the sense that at first, despite kinship with certain nations, we stayed out of it. Roosevelt was very much in favor of getting us involved to support the beleaguered British, but again we didn’t feel like it was our business. Then the Japanese attack Pearl Harbor and their ally Germany declared war on us.
As you can see, we generally try to stay out of European wars until our interests compel us to enter.
In World War One they didnt want to be involoved but ended up getting caught up in it and joining after reciving a telogram that was messed with from Germany and then with the sinking of a ship by u-boats that germany had that contained United States people on board. Once the US joined the allies were able to defeat the central powers and the war was over.