AMSCO APUSH Practice Exam 2025 - Free AP U.S. History Practice Questions and Study Guide

The primary cause of the U.S. declaration of war in April 1917 can be attributed to Germany's violation of U.S. national rights, particularly through its unrestricted submarine warfare policy. This policy threatened American vessels and lives, particularly highlighted by the sinking of ships like the Lusitania in 1915, which resulted in the deaths of American citizens. Despite previous warnings and diplomatic efforts to maintain neutrality, Germany's continuous aggression and blatant disregard for neutral shipping rights heightened tensions. The interception of the Zimmermann Telegram, in which Germany proposed a military alliance with Mexico against the United States, further fueled public outrage and solidified the perception that Germany posed a direct threat to U.S. sovereignty and security. This series of events ultimately led President Woodrow Wilson to ask Congress for a declaration of war, framing it as a necessary action to defend American rights and uphold international law.