Actually, WW1 is what led to the demise of the UK, not WW2. WW2 was just the denouement. The generations that were lost on the Somme and Flanders crippled the empire.
WWI was the cause of the demise of pretty much all of Europe, directly resulting in heavy generational damage to Western Europe, kickstarting the communist revolution in Russia (which might have been tolerable in and of itself under Leninist or Trotskyist rule, but was then classifiable as "demise" once Stalin was in power), and (in the end) imposing excessive punishments on Germany post-war that caused severe economic problems and spurred the desire for a return to glory - one promised (and briefly realized) by the Nazi party.
