Question

What was one of the events that led to U.S. involvement in WWI?

U.S. troops were captured after they were sent on a peace seeking mission.
Germany sunk the Lusitania and other American ships.
Germany invaded waters too close to the United States.
The United States sunk German U-boats, aggravating tensions between the two countries.

Why did Germany sign an armistice in 1918?

Germany's allies collapsed and retreated.
The armistice was a temporary halt to the war and Germany needed to refresh troops.
Turkey was the only ally left and that wasn't enough to fight.
Germany wanted to allow the Central Powers time to regroup.

Answer 1

1) Germany sunk the Lusitania and other American ships.

Germans knew that the US was supporting the Allies through passenger ships which were loaded with ammunition and other wartime needs.

After a while, Germans decided to apply the "Unrestricted Submarine Warfare" which means to sink every ship they encountered.

Following to this decision, the US warned Germany to withdraw this policy. Germany resisted and this was the one of the biggest reasons why the US entered the war.

2) Germany's allies collapsed and retreated.

After the surrender of Ottoman Empire and Austria-Hungary, there was not much left for Germans.