World War II
Battle deaths: 50,000,000 
Nation(s) involved and/or conflict territory [note]
Australia, United Kingdom, United States, France, Italy, Germany, Denmark, Netherlands, Soviet Union, Japan, Norway, Finland, Spain, Poland, Austria, Spain, Greece, China, Canada, New Zealand, India, South Africa, Belgium, Czech Republic, Ethiopia, Mexico, Hungary, Romania, Bulgaria, Finland, Thailand, Iraq, Costa Rica, Yugoslavia
The war was significant in that it was the first war in which air power was a significant factor. Indeed, the first combat operation in World War II was a German bombing attack against Poland, while the last combat operation was the American atom bomb attack against Nagasaki. The war also saw the re-emergence of the United States from its isolationism, the destruction and rebuilding of Germany and Japan into major industrial powers, and the emergence of the United States and the Soviet Union as global superpowers. The war also directly led to the United Nations, which was founded by the victorious Allies in order to prevent such a large and destructive conflict from ever again happening.
SOURCES: FATALITY DATA
Notes on fatalities
 Battle deaths: Around 50 million deaths is a commonly cited number and I have chosen to use it here as well as the best estimate. It includes military and civilian battle deaths and excess deaths to some extent. This estimate has to be reviewed.
wikipedia.org/wiki/World_War_II_casualties puts the numbers at 22-30M military deaths and 38-50M deaths because of military activity and crimes against humanity. Correlates of War, Inter-State War Data v4.0 has 16,634,907 battle deaths. Matthew White has 65,5 Million including excess deaths.
NOTE ON NATION DATA
NOTE! Nation data for this war may be inconlusive or incomplete. In most cases it reflects which nations were involved with troops in this war, but in some it may instead reflect the contested territory.