With what?
The U.S. military in 1940 was somewhat less than robust.
Good navy, albeit in the early stages of replacing the older Treaty hulls, AAF was just starting to get some useful aircraft, and the Army was under 270,000 men, and that was a HUGE increase from 1939, when the entire active Army was under 190,000. The entire U.S. military had a smaller ration strength in 1940 than the Marine Corps did all by itself in 1945.
The U.S. added 1,200,000 troops to the active Army ranks in 1941, with another 1.6 million in 1942, and 3.8 million in 1943. Overall the U.S. military went from 334,000 active troops in 1939 to 12 MILLION+ under arms in 1945 (this figure does not include KIA/WIA discharged due to invalidity/mustered out).
The expansion of the U.S. Army between 1939 and 1942 is one of the more miraculous parts of the entire U.S. effort in WW II.