It wouldn't have worked.
Firstly, WW2 caused the second highest amount of casualties in American history. Only the Civil War had more.
405,399 Americans dead
670,846 Americans wounded
30,314 Americans missing
1,106,559 Americans are unable to fight.
139,928,165 was the population size in 1945
In 2000 40% of the population was 18 to 44 (usually capable of military combat).
Let's project 40% onto 1945 population size 55,971,266
Now let's half that to exclude women; 27,895,633
Subtract WW2 casualties = 26,879,074 men capable of fighting a war.
Operation Downfall projections assume 1.2 million American casualties in the invasion of Japan. (Note: this was to be the largest invasion in human history, but Japan surrendered after being nuked.) Project that 1.2 million casualties onto an invasion of any powerful country at the time and the US would run out of men very quickly.
The United States could have exercised Plan Rubber and have taken Brazil, but that's about it. (Note: This invasion plan was crafted in case Brazil sided with the Nazis, but they didn't so Plan Rubber was scrapped.)
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