What reasons did the u. s have to join ww1

There were a few good reasons as to why the US left their promise of neutrality to enter WW1. One reason was the Lusitania, which was a passenger cruise ship full of American citizens coming from Great Britain. It was sunk by the German U-boats, who thought that the boat contained British passengers.

Another reason for the entry into WW1 was Germany's unrestricted submarine warfare. After many warnings from the US not to test their submarines in certain regions, the Germans didn't care about that.

Last and most important reason would be the Zimmerman Note. Arthur Zimmerman, the German Prime Minister at the time, sent a telegraph to Mexico. The message was Germany attempting to form an alliance with Mexico to aid Germany in neutralizing the US in WW1. As a reward, Germany would give Mexico back Texas and any other land they lost to the US. Fortunately, Britain intercepted the telegraph. It never reached Mexico, but instead reached the front desk of the White House, in President Woodrow Wilson's hands. This was the last straw for him and he declared the US's entry into WW1