Question:

How did German policy from January 1917 onward cause the United States to declare war?

In January 1917 Germany reinstated its policy of unrestricted submarine warfare. Any ship, including unarmed passenger liners, was susceptible to attack without warning. In March, British intelligence intercepted a telegram from Germany to Mexico hinting at a possible alliance between the two countries where Germany would aid Mexico in reclaiming lands lost (viz. Texas, New Mexico, and Arizona) if Mexico would fight alongside Germany. Soon after, as a demonstration of its naval policy, Germany sank five unarmed American merchant ships. In response to these events, on April 2, 1917, President Wilson asked Congress to recognize the existence of a state of war between the United States and Germany.

Germany instituted its own blockade in response to the British blockade in 1914. After the sinking of a third unarmed ship with American passengers in 1916, Germany issued the Sussex pledge, promising to not sink any more merchant or passenger ships without due warning. The pledge was kept until January 1917, when the original policy of unrestricted submarine warfare resurfaced. Germany felt that by cutting off supplies it could end the war on its terms before the United States could react. Shortly afterward, the United States cut off diplomatic relations with Germany, a step preparatory to war.

In March 1917, American newspapers reported of a secret offer from Germany to Mexico. British intelligence had become aware of a telegram that proposed a Mexican alliance with Germany in exchange for a German pledge to help Mexico recover former lands (viz. Texas, Arizona, and New Mexico). This note, known as the Zimmermann Telegram, aroused American nationalist furor and convinced President Wilson that Germany expected war with the United States. Later that month, Germany sank five merchant ships, a strong demonstration of its submarine warfare policy.

Though President Wilson won reelection on a campaign that touted his commitment to neutrality, he asked a special session of Congress to declare war on Germany on April 2, 1917. In addition to the revelation of the Zimmermann Telegram and the sinking of the merchant ships, the Russian Revolution occurred, deposing the czar and creating a republic. Since Wilson wanted to fight the war with the purpose of making the world safe for democracy, it bothered him that Russia, one of the Allied nations, was ruled by an autocrat. This barrier to U.S. involvement was removed with the uprising.