At the beginning of WW2, the U.S. Army was smaller than the army of Portugal.
When the war starts, the U.S. military was smaller than Portugal's. It was only used in places of tension like the Caribbean or South America, but was incapable of larger campaigns, as an invasion of Mexico or Canada. Americans were hesitant of military engagement. Most Americans did not want to get into World War II because their efforts in World War I never saw a true resolution.
After Pearl Harbor on Dec. 7, 1941, the United States had to go to war very quickly. The relatively small American forces found themselves against Japan, which had been fighting in China since 1931, and Germany, which had been fighting in Europe since 1939. Yet, in less than 2.5 years, the United States was winning battles in North Africa, Italy, and Guadalcanal. Despite their weak start, the American Army did not lose vast the number of soldiers (compared to other countries) due to their reliance on airpower, artillery, and commanders that didn't take unnecessary risks.