Yes. The United States military fought on the side of the Allies in World War I. On the Western Front of this war, where the United States fought, the primary fighting was done by way of trench warfare.
The two sides who fought in the Civil War were the Confederacy, which were the 11 Southern United States that seceded from the Union and the United States which were the rest of the states in the Union.
That depends on the reference point, as both Germany and the United States were fighting on two fronts during the war. The Germans would say the United States fought on the Western front. The United States fought on the European front and the Pacific front.