In reality we didn't enter the European war to liberate the concentration camps. We entered for far larger reasons of national security. Nice we were able to save a lot of innocent prisoners in the process, but that was, and should have been, a byproduct not a casus belli. Other wise we should have attacked the USSR and freed up the Gulags, right? Where does it end?
Exactly.
BZZZT. Wrong question.
The correct question is, where does it begin?
Long, complex story: but usually, what happens is that these things can be stopped (say, by the US, but in the past it was other Western Powers) at a cost in lives which is VERY LARGE for peacetime, but VERY SMALL compared to the number required to stop them once the can is kicked down the road.
What if France and Britain had opposed Hitler over (say) the Sudetenland?
What if Truman had nuked Stalin over Iran in '46?
What if John Kerry and Jane Fonda had been executed by firing squad for treason in the Viet Nam era?
Food for thought.
Cheers!