You have to somehow calm Japan down, so that they don't drag the US into the war, as once that happens it's a guaranteed game over for the Axis in relatively short order. One way to do it would be to have the Soviet-Japanese border clashes that culminated in the battle of Khalkin Gol escalate into a full-on Soviet invasion of Manchuria and Korea that drags on and on.
Then you need for Germany to start the war in Europe (ergo Hitler), but also for Germany to not invade the USSR after conquering France (so probably have Hitler killed down the line, maybe while he's in Paris).
Probably also required is a much more isolationist US President.
Whether or not Italy joins is sorta immaterial.
Thus, you'd have a constant naval and air war as Britain is to weak to invade continental Europe and Stalin is too cautious to stop trading with Germany. Eventually, given that the scenario requires an Allied victory, you need to have the Tube Alloys project bare fruit for Britain. After Germany eats dozens and dozens of nukes and the British finally land, the Soviets sense their moment and invade as well.
So probably sometime in the early-to-mid fifties