The Soviet Union is the successor to the Russian Empire of the tsars. The legitimacy of government in the great multinational empires of the past was based on the duty of obedience to the reigning monarch, deriving from the divine right of kings. The legitimacy of the Soviet empire is based on a doctrine that attributes universality to what its spokesmen consider to be the greatest event in human history, dividing history in two in a more radical sense than the birth of Christ or the Hejira of the prophet Muhammad—the Great October Socialist Revolution. The Soviet constitution gives every republic the right to secede from the USSR. The Ukrainians, with a population of more than forty million, make up the second most numerous nations in the Soviet Union. Soviet literature on this subject is replete with rhetoric and also with semantic disputes of Talmudic subtlety relating to the definitions used in Stalin's famous and partly discredited article.