>>So, what exactly was achieved by the millions killed in that war

Speculation is an inexact science but the forces that shaped Nazism would still have existed in a victorious Germany. With an improved polity and better economics, there would ultimately have been another world war, but with a far higher chance of going the other way.

I'm old enough to have known many who fought on the Western Front, but for what it's worth, they widely agreed that they had done something worthwhile.