Japan should've never attacked pearl harbor. It set off the dominos that led to the hell we live in today. Most likely the US would've entered the war sooner or later but a year's delay could've been decisive. Without the massive US aid, its debatable if the British could've held on on North Africa or the Soviets could've defeated the German summer offensive in 1942.

Those would go onto have snowball effects later on. The US took a year to mobilize fully. Without huge US intervention in Italy and North Africa, Germany would not be forced to divert a fifth of its eastern front fighting strength and the Soviets thus would not have been able to push west in 1943. With US air power arriving a year later, Germany would not have been bombed in 1943, allowing the Germans to fully mobilize war production by 1944 as they had originally intended.

The ruhr bombing in early 1943 prevented Gemran monthly aircraft production from rising from 1200 to 2000 in that year. Plus German production in all weapons categories would've greatly accelerated and would stand higher in 1944. Thus slowing any potential soviet advance.

All in all, its debatable if the Soviets could've advanced at all had the US entered the war one year later.

In fact, without the Japanese attack, the US might not have entered the war at all. The public was deeply set against a conflict. And without American intervention in the war, the allies would've lost. Britain would've been starved into submission and the Soviets would've fought Germany to a standstill.