What was a major factor that led the United States to inter world war 1

There were two main reasons the United States entered WWI. It's allies were under attack. They were going to have to enter regardless because of countries such as Britain being attacked. The immediate cause is the Zimmerman telegram. Germany tried to get Mexico to attack the United States. This left the country no choice but to declare war on Germany and enter WWI.

The major factor was that Zimmerman Telegram and Germany's announced intention to resume unrestricted submarine warfare.