1) - entering a war is always connected with causalities, and not having entered this war would have saved American lives.
2) one could argue that there was no immediate thread to the US - in other words, it was "not US's war"
3) the war was a big financial blow to the country
4)one could argue that the US's engagement allowed to force Germany to capitulate (rather than reach some kind of compromise) - and the terms of this capitulation can be claimed to make Nazi Germany possible (Germans grew flustrated with the poverty they brought)