During the time of the war, the Allied and Axis Powers were the primary belligerents, but the United States still had their "hands-off" stance with international affairs. Though the United States did not directly fight in the war until 1918, they did assist their allies in Great Britain by secretly sending supplies by sea vessels across the Atlantic.
In 1915, a German U-Boat sank the RMS Lusitania, a British ocean liner, carrying British and American citizens to Liverpool. This was ultimately the determining factor for America to join the war, as it put America and Germany on thin ice. America ordered Germany to stop their U-Boat campaign, and when the commenced it again, America entered the war to stop them once and for all.
