I was curious. We always hear about how in Europe in WWI a huge percentage of the population died, but then in WWII an even bigger part did. How did Europe manage to repopulate so quickly in 20 years, especially since the birthrate had slowed by this time?
You're overestimating the population loss. Britain lost around two percent of its total population during the war. France lost just over four percent and Germany just under four percent. Tragic and horrifying, yes, but not large enough to totally disorder society.