World War I

The United States originally had a policy of isolationism, avoiding conflict while trying to find peace. This resulted in increased tensions with Berlin and London. When a German U-boat sank the British liner Lusitania in 1915, with 128 Americans aboard, U.S. President Woodrow Wilson vowed, "America was too proud to fight" and demanded an end to attacks on passenger ships. Germany complied. Wilson unsuccessfully tried to mediate a settlement. He repeatedly warned the U.S. would not tolerate unrestricted submarine warfare, in violation of international law and U.S. ideas of human rights. Wilson was under pressure from former president Theodore Roosevelt, who denounced German acts as "piracy". Wilson's desire to have a seat at negotiations at war's end to advance the League of Nations also played a significant role. Wilson's Secretary of State, William Jennings Bryan, resigned in protest of the President's decidedly warmongering diplomacy. Other factors contributing to the U.S. entry into the war include the suspected German sabotage of both Black Tom in Jersey City, New Jersey, and the Kingsland Explosion in what is now Lyndhurst, New Jersey.

In January 1917, after the Navy pressured the Kaiser, Germany resumed unrestricted submarine warfare. Britain's secret Royal Navy cryptanalytic group, Room 40, had broken the German diplomatic code. They intercepted a proposal from Berlin to Mexico to join the war as Germany's ally against the United States, should the U.S. join. The proposal suggested, if the U.S. were to enter the war, Mexico should declare war against the United States and enlist Japan as an ally. This would prevent the United States from joining the Allies and deploying troops to Europe, and would give Germany more time for their unrestricted submarine warfare program to strangle Britain's vital war supplies. In return, the Germans would promise Mexico support in reclaiming Texas, New Mexico, and Arizona.

After the British revealed the telegram to the United States, President Wilson, who had won reelection on his keeping the country out of