How Did a German Telegram Drag the United States Into the First World War and When?

In 1917, the British intercepted a cable from the German foreign minister to their Mexican ambassador.

It proposed an alliance whereby the Mexicans would reacquire Texas, Arizona, and New Mexico if that country would join Germany in an attack from the south on the neutral Americans.

The British made the telegram public on March 1, and the outcry forced the United States into the war a month later.