It came about because the most convincing argument Hitler made to come INTO power was the fact that, according to him at least, it wasn’t the German people who lost WWI it was the Jews, the Communists, and all the others who sold Germany down the river.
Hitler’s speeches constantly drove home the fact that, again according to him, Germany could’ve won WWI had it not been for traitors who sold the country out and negotiated a treaty. And then what a treaty it was, according to him. The Treaty of Versailles, was a very convenient scapegoat for all of Germany’s post WWI woes
Without the help of the USA, the Allies in WWI could’ve at best gotten a stalemate, and a negotiated settlement would’ve put Germany, at worst back where it had started, probably left it better off.
After the Germans negotiated “Versailles” (or in Hitler’s opinion, had it dictated to them) they attempted to reneg on it, only to find by that time the British Navy had a stranglehold on Germans food and would simply starve Germany to accept it.
So while the war went on, the Allies were demanding the Germans must not simply sign a paper admitting responsiblity for the war, as was done in WWI, the German people had to completely give up and have their will broken.
Add to this was an organized effort to “de-Nazi” people, and mass movements of German populations out of non-German areas. Now-a-days, that would be poltically incorrect but it worked, very well.
Japan on the other hand was a bit of a difference. Only China and the USA were any big players in the Pacific War. The British helped through their navy but most British action involved India (part of the Empire) and defending it from Japan. The USSR was not even involved right till the end.
Japan did get a few conditions like the ability to decide for itself whether or not the emperor stayed on.
This is a bit oversimplified but you can see it today as the Japanese still have refused to come to terms with their war crimes in WWII while the Germans admitted it and moved on.
So unconditional surrender was aimed at assuring that defeated people KNEW they were defeated. In war to win you have to break a person’s will, without this it just goes on and on. This is why you still have problems in Israel / Palestine. This is why the Napoleonic War went on so long. The Boer War was a guerrila war that was won by the British through the use of concentration camps. Cruel but it worked.
Was it effective? No, France surrendered even though they had an agreement with Britian to fight on. (This was different from the Unconditional Surrender of the Allies but still). Stalin on twice tried to open negotiations with Hitler after 1942. Hitler thought he could win and laughed them off. “Unconditional Surrender” was nothing to Stalin.
Too many people look at WWII through today’s politically correct ways and don’t fully see, WWII was fought because negotiations and promises were not kept and were a joke.
Look at Iraq, Afghanistand and Lebanon, these are wars fought ON THE ENEMY’S TERMS. You can’t win a war like that.
The object of WWII was never to save lives, it was to bring total defeat on the enemy so he could never do it again. When the USSR is losing tens of millions of people through combat, disease, famine and forced labor a few more lost because of “Unconditional Surrender” hardly matter.