Why was the Zimmerman telegram a reason for the United States to join world war 1

Zimmermann sent the telegram in anticipation of the resumption of unrestricted submarine warfare by Germany on 1 February, which the German government presumed would almost certainly lead to war with the United States. The Zimmermann Telegram helped turn the U.S. public, already angered by repeated German attacks on U.S. ships, firmly against Germany. On April 2, President Wilson, who had initially sought a peaceful resolution to World War I, urged immediate U.S. entrance into the war.

rosevelt. 1973. "slaves everywhere"