It gave supplies to the Allies
Stalingrad cost Germany 23% of its entire army. They never stood a chance after that big of a lose.
In World War I, USA helped Britain in the war however they joined in the war 1917 and didn't involve a lot that was part of the reason why they didn't want to punish Germany after World War I. They...
Please have a look at the related question below. It is not at all obvious what would have helped.