Why did Americans agree to
participate in the UN after World War II when
many had rejected participation in the League of
Nations after World War I?

This was because of a multitude of factors, but primarily due to The Bombing of Pearl Harbor.

Explanation:

During the end of the 1930's and early 1940's we had the Neutrality Act, which basically said no interfering with other's wars. We still sold good to both the Axis and Allies, but they had to pay immediately and had no pick it up, no credit, no shipping. Japan was allied with the Axis during 1941, Japan decided to bomb Pearl Harbor. This was an attack on American Soil, our patriotism basically went into effect and since we were sided with the allies, we decided to join them in the European Front.

"germany's geographic location was an advantage when it was on the offensive in the war because it was situated next to an extremely wide river, the rhine, which was difficult for the allies to cross."