Signing a defense agreement between the United States and Greece during a brief ceremony at the Greek foreign ministry on a sunny Saturday afternoon in Athens, U.S. secretary of state Mike Pompeo initiated a new American strategy of contesting China’s mercantilist commercial expansion. While the agreement did not mention China, in geopolitical terms it marks a fundamental challenge to China’s ambitions in Greece, the Mediterranean, and the European Union.

The signing took place just a few miles from the port of Piraeus, which abuts the Greek capital and is the major symbol of China’s mercantile ambitions in the West. Chinese premier …