The British government accomadated Hitler's regime until British interests were threatened, then it decided to go to war. It was nothing to do humanity coming together, or closing down the death camps, or democracy (though that was what inspired many to fight).

Humanity is divided into classes with opposing interests, with the productive resources controlled by small section of society and the rest enslaved to that economic power. These material differences cannot be reconciled. Hitler was bankrolled by German industrialists to gain more resources by building an empire (their's had been taken in WWI). The British ruling class, and later the American, were not fighting Hitler for any noble reason but to preserve the productive resources under their control. For the vast majority who don't own productive resources it matters not one jot whether the resources are owned by Britons, Americans, Germans or Indian - the class antagonism remains. As long as productive resources are controlled by the few they will fight over them (but they themselves don't do the actual fighting). Working people should struggle only against the ruling class to gain the power and resources that they have and use it democratically, equitably and sustainably (the ruling class will never do this) and put an end to wars. They are outnumbered by billions so it should be the easiest struggle ever.