The concept of universal empire — that a single ruler should legitimately govern all the known world — appears independently in multiple ancient civilizations and shaped political thought for millennia. Alexander the Great's explicit claim to rule Asia as successor to the Persian king was a new kind of political ambition for a Greek. The Roman claim to rule the orbis terrarum (circle of the earth) was embedded in Augustan ideology and survived as rhetoric even when the actual empire covered only a fraction of the world's land area.

Chinese imperial ideology had its own version: the Son of Heaven ruled Tianxia (all under Heaven), and tributary relationships with neighboring peoples were understood as recognizing this universal sovereignty rather than as international relationships between equals.

The interesting question is what happens when these universalist claims meet each other. The Han-Roman world didn't interact directly enough for the question to become acute. But the later history of Eurasian political thought is largely about the collision and negotiation between different universal claims — Roman/Byzantine, Islamic caliphal, Mongol, Chinese.

The ancient foundations of universalist political thought are worth understanding if we want to understand why so much subsequent political conflict took the form it did.