ABOARD THE USS MILIUS—The chase began just after lunch.

An American P-8 surveillance plane had spotted three ships huddled together in the East China Sea. About 80 miles northwest, the USS Milius and a Japanese warship were following a tanker blacklisted by the United Nations for smuggling oil into North Korea.

The Milius broke away, hoping to catch other ships engaged in the illegal activity. Powered by four engines similar to those used in Boeing 747s, the destroyer raced to the site, where two of the vessels were suspected to be secretly transferring petroleum products to a tanker bound for North Korea.

The three ships had all switched off transponders signaling their locations to other vessels, a common tactic for ships engaged in illicit trade.

As the ships came into view two hours later under a bright, clear sky, the Milius picked up excitable, broken ship-to-ship radio chatter in Chinese.