I am well aware that this may cause some harsh feelings to pop up. And quite frankly, this is pretty macabre. But bear with me.
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We all know that World War II was bad. Massive war crimes on all sides, millions of deaths caused by a cold-blooded system or simple war incompetense.
Although this question is rather macabre, I would like to ask: what would be the worst-case scenario in terms of damage caused, both to property and finacially, along with the amount of blood spelt?
Something that may be possible: the Eastern Front stalemates out for an extra year or two, causing the Soviets to start crumbling around 1944.
The US enters the the European War, but in both Europe and Asia, a combination of mediocre leadership and butterflies from the Eastern Front cause their advance to be delayed, but not stopped.
Eventually, the Soviets start making counteroffensives, but due to losses caused by the prolonged front, by the time they retake Kiev, basically everything between Odessa and Riga is burnt out ruins.
German losses are also higher, however, due to the prolonged Eastern Front. Weakened, the US launches D-Day about a year later, and reaches western Germany by the winter of '45.
Japan gets nuked as OTL, but doesn't surrender. So Coronet and co. are launched. Although the US manages to defeat Japan, they've suffered greatly, along with the Japanese.
Falling apart on both fronts, Germany gets nuked a few times. A coup replaces Hitler, and they surrender, when the Wallies are on the Elbe.