World War I, also referred to as The Great War, was a conflict fought mainly in Europe on Earth during the early 20th century.
The Central Powers – the Austro-Hungarian, Ottoman, and German Empires – were opposed by France, Russia and their allies, the Italian, British and Japanese Empires. In 1917, the fourth year of the war, Russia was embroiled in a series of revolutions and sued for peace with the Central Powers, while the USA entered the war against them.
Spock's figure of six million deaths falls short of the sixteen million casualties of World War I, but may be a reference to seven million civilian deaths or the number of deaths as a result of "despotism".
Actors Paul Fix and Ian Wolfe are the only two known Star Trek performers who were veterans of the First World War.