The United States had adopted an isolationist view, not wanting to be involved in affairs across the ocean that were not directly related to our national security.
The US declaration of war by Congress finally did occur on April 6, 1917.
The reasons that led to declaring war:
1. In January, 1917, Germany had resumed its policy of unrestricted submarine warfare. Germany had halted its attacks on non-military vessels (which it suspected of carrying military supplies) after the furor over the sinking of the Lusitania in 1915. But now Germany was resuming attacks by its U-boats.
2. In February, 1917, the "Zimmerman Telegram" was intercepted by British intelligence and shared with the US. Germany's foreign minister, Arthur Zimmerman, had telegraphed an offer to Mexico's ambassador seeking Mexico's support in war vs. the United States in exchange for getting land back from the US.
3. On April 2, 1917, President Wilson made a powerful speech to Congress in which he argued that the nation needed to enter the war "to make the world safe for democracy." Wilson's speech was powerfully convincing, and four days later, Congress declared war.
