After having had it for some time, the United States released the contents of the Zimmerman Telegram which, as we have been following, proposed a German-Mexican alliance in the event of an American entry into World War One.
American public opinion was becoming increasingly hostile to Germany in 1916 and 1917 and it was already hostile to Mexico given the numerous border problems that had being going on for years and the strained relationship with Carranza. The release of the telegram was one more event that helped push the United States towards going to war with Germany. In some ways, the telegram confirmed suspicions that were already out there as presence of German military advisors in Mexico was well known and they had taken an active role in advising Mexico's prevailing army. They had even been in one instance in that role in which Mexican troops had directly engaged American troops. In recent weeks there's been speculation in the press about German activities in Mexico and Carranza's relationship to Germany. So, while Zimmerman's suggestion seems outlandish to us in retrospect, to Americans of 1917 it would have seemed to confirm what was already widely suspected, but with details far more ambitious than could have been guessed at previously.