Answer:
Sinking of the Lusitania
Step-by-step explanation:
Since Washington's farewell address, many Americans supported isolationism, especially in Europe.
American Isolationism
In early 1900s, most Americans thought that WWI was a European issue and that America had no reason to involve itself. People wanted to protect Americans and stay out of European entanglements. Nevertheless, America continued to send ships in and out of Europe. America had declared neutrality, and Germany had agreed to not sink American ships. However, in 1915, Germany sank the American passenger ship known as the Lusitania. America took this as a personal attack and joined WWI on the side of the Allies in order to fight against Germany.
American Involvement
After the sinking of the Lusitania, America helped the Allies by sending troops and supplies. Since no fighting occurred on American land, all of their factories were still running. This allowed them to supply the Allies with war materials. Additionally, America was involved in the negotiations that helped end the war.