The United States entered the war because of the Germans' decision to resume the policy of unrestricted submarine warfare, and the so-called "Zimmerman telegram," intercepted by the British, in which Germany floated the idea of an alliance with Mexico. Unrestricted submarine warfare, a desperate effort to counter the British blockade of Germany, would lead to the sinking of American merchant ships heading for England, and had been employed by the Germans before. They abandoned it in the face of US pressure earlier in the war. Its resumption was enough to cause Woodrow Wilson to renounce his stated position of neutrality, as his war speech to Congress demonstrates:

The new policy has swept every restriction aside. Vessels of every kind, whatever their flag, their character, their cargo, their destination, their errand, have been ruthlessly sent to the bottom without warning and without thought of help or mercy for those on board, the vessels of friendly neutrals along with those of...

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