The McClellan saddle was adopted by the U.S. Cavalry in 1859. It was designed by future general, George McClellan after a tour of Europe where he studied various cavalry saddles. He based it on a Hungarian saddle used by the Prussian cavalry. It’s also a modification of the Mexican Tree saddle that was popular at the time. During the early years of the Civil War Confederate cavalry used a similar saddle called a Walter Jenifer but as war dragged on their horses lost weight and the boney withers were injured by it so they went to the McClellan.
Modifications of the McClellan were in use until WWII and are still used for ceremonial purposes and mounted police. The advent of tanks, barbed wire barricades and automatic weapons during WWI foresaw the end of mounted troops. The last time U.S. cavalrymen saw action was on the Philippines in 1942.