Xi Warned of the ‘Thucydides Trap.’ What Is It? China’s leader reached for Greek history to warn the United States of what can happen when a rising power meets an incumbent one.

“The idea is that when an established, great power is met with a rising power, conflict between the two is certainly likely if not inevitable,” said Daniel Sutton, a classicist at the University of Cambridge who studies Thucydides, on Thursday.