LIST OF WARS: DETAILS
World War I
Battle deaths: 10,670,868 
Nation(s) involved and/or conflict territory [note]
Australia, Germany, France, United Kingdom, Italy, Russia, Greece, Hungary, Turkey, United States, Austria, Andorra, Belgium, Brazil, China, Costa Rica, Cuba, Costa Rica, Guatemala, Haiti, Honduras, Japan, Liberia, Montenegro, Nepal, Nicaragua, Panama, Portugal, Romania, Russia, San Marino, Serbia, Thailand, Canada, Australia, Malta, New Zealand, South Africa, Philippines, Puerto Rico, Azerbaijan, Bulgaria, Austria-Hungary, Dervish State, Jabal Shammar, Ottoman Empire
SOURCES: FATALITY DATA
Notes on fatalities
 Battle deaths: en.wikipedia.org/wiki/World_War_I_casualties - military deaths (9,722,620) and civilian deaths caused by direct military action (948,248). Excess deaths are estimated to 5,893,000 including the Armenian Genocide. No low or high estimates.
Correlates of War, Inter-State War Data v4.0 has the number 8,578,031.
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.