What made world war 2 the bloodiest conflict in human history?

There are multiple answers to this question. Some ideas/theories include:

  • The war's length
  • it involved quite a few countries
  • many people died not athrough direct violence but side effects of war (homlessness, destroyed food supply, ect.)
  • the hollocaust is sometimes included to say this
  • us nuking japan

also as a side note the Aerican Civil war resulted in the largest loss of American lives.