It was the British. They intercepted the Zimmerman note where they offered to help Mexico reclaim territories that it lost to the United States if they support Germany in the war against the Allies. Before this development, the U.S. tried to keep itself out of the war. But when the British revealed this info along with the sinking of the Lusitania caused by unrestricted German warfare in the Atlantic Ocean, America joined the war on the side of the Allies.