The United States had never maintained a large standing army, and it wouldn't until after World War II. Many believed that America was kept safe by her oceans, and that a large military presence was undesirable. Even as the Great War unfolded, the United States still resisted building up its military until it was absolutely necessary. As a result, America was underprepared for the war, and slow to rise to its potential.
Warfare had changed dramatically since America's last war. The daring cavalry charge was no longer useful. Battles now were won and lost with vehicles like tanks and aircraft for the first time in history. America had not only to enlist and train soldiers, it had to learn how to fight a new kind of war and build the machines this new kind of war required.