What important events led to U.S. involvement in World War I?

In January 1917, after the Navy pressured the Kaiser, Germany resumed unrestricted submarine warfare. Britain's secret "Room 40" cryptography group had decrypted the German diplomatic code, and discovered a proposal from Berlin (the famed Zimmermann Telegram) to Mexico to join the war as Germany's ally against the United States. The proposal suggested that Mexico should declare war against the United States and enlist Japan as an ally; this would prevent America from joining the Allies and deploying troops to Europe, which would give the Germans 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 Americans, Woodrow Wilson was still attached to neutrality but released the captured telegram as a way of supporting his proposed plan to arm American merchant ships. After submarines sank seven American merchant ships and the publication of the Zimmerman telegram, Wilson called for war on Germany, which the U.S. Congress declared on 6 April 1917.

The United States was never formally a member of the Allies but became a self-styled "Associated Power". America had a small army, but it drafted four million men and by summer 1918 was sending 10,000 fresh soldiers to France every day. Germany had miscalculated that it would be many more months before they would arrive or that the arrival could be stopped by U-boats.|||Germany's use of unrestricted submarine warfare was probably the biggest issue. The U.S. has always had a really protective streak. So when the Lusitania was sunk, they felt morally outraged enough to join the war.|||Lovely Lady 0927 has the details above - unrestricted submarine warfare was the key factor (more so than the Zimmerman telegram since few Americans were truly concerned about a Mexican invasion of the States).

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What important events led to U.S. involvement in World War I?

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