Question

why was the united states unable to remain neutral during world war 1?

Answer 1

First the Zimmerman Telegram in which German ask Mexico to declare war against the U.S and U-Boat that sink the U.S navy cause the U.S to enter WWI to stop German

Answer 2

In fact, it did remain neutral for almost all of the war, until 1917, in part because a number of Wilson's domestic allies (Progressives, unions and immigrants, prohibitionists) were strongly opposed to war with Germany, in part because Wilson's domestic enemies (Republicans in Congress and former President Roosevelt) were strongly in favor of war with Germany. Had Wilson supported war, he would have fractured his own coalition and appeared to endorse the position of his opponents in the 1916 election.

After the election, however, Wilson had a freer hand to act, and it is likely his own internationalist-interventionist tendencies favored "policing" Europe anyway. Furthermore, the desperate Germans decided in early 1917 to stop avoiding sinking (by submarine) American-flagged ships who were trading both supplies and weapons with the Allies, in defiance of Germany's declaration of blockade. They did this knowing it would likely enrage American opinion sufficiently to bring the United States into the war, because they hoped to finish the war before the Americans could arrive in force. (In this they were deluded.) The simultaneous exposure of the infamous Zimmerman telegram also helped Wilson rally the country behind a declaration of war.