World War Two was, and thankfully remains, the worst war in the history of humanity. It resulted in the deaths of between 50 and 80 million people, most of them civilians and many of them murdered in the Holocaust, the first industrialized genocide in history.
What made World War Two so awful was the fact that it was the first major conflict fought after the war machine of each nation involved became truly a machine: the armies were fully mechanized, from the small arms they carried to the tanks they rode into the fighters and bombers that fought overhead. Scientists raced to develop and deploy chemical, biological, and eventually nuclear weapons designed to maximize loss of life and crush the enemy into submission.
The War was thus taking place exactly at the confluence of two things: the willingness of fascist regimes to fight a bloody ground war of conquest with a full mobilization and damn the fact that this meant overwhelmingly civilian deaths, and the invention of weapons horrible enough to carry out this vision. Just as this clash enabled the Holocaust to produce a new kind of mass murder, World War Two became a new kind of industrial horror.
As a result, the battles of this war had death tolls that sometimes surpassed even the tolls of World War One. While one army or other might call itself the winner, it usually meant leveled towns, murdered civilians, and refugees fleeing. Still, someone had to be the winner - so let's see if you know who it was!