World War II cost 62,000,000 lives.
World War I cost 66,000,000 lives if you count those caused by Spanish flu as a result of the close contact of different nationalities with different immunities to viruses.
Before these, though, the Mongol invasions of Europe and Asia in the 1500s cost 60,000,000 lives - which is a much more significant proportion of the world's population at the time.
