When the US joined the war the allies were losing BADLY. The germans were at the gates of moscow after having steamrolled every nation they fought in Europe, Rommel was still running roughshod over the Brits in Egypt and Libya, and Japan within weeks of pearl harbor had steamrolled most of the allied holdings in the pacific capturing the phillipines, singapore, much of papua new guinea, and borneo.
Hitler repeatedly ignored sound advice from his generals and distracted the german military from more important objectives in favor of winning propaganda victories. Stalingrad, for instance, was not actually strategically important. Hitler wanted it taken because he believed losing a city named after Stalin would be perceived as a huge blow to Stalin.
The US won virtually no war spoils and in fact ended up spending absurd amounts of its own money to rebuild a devastated Europe and japan after the war