David Wise is a writer and historian of intelligence and espionage. His most recent book is “Tiger Trap: America’s Secret Spy War with China.”

IN 1995, a middle-aged Chinese man walked into a C.I.A. station in Southeast Asia and offered up a trove of secret Chinese documents. Among them was a file containing the top-secret design of the American W-88 nuclear warhead that sits atop the missiles carried by Trident submarines.

He told a story to the C.I.A. that was so bizarre it might just be true. He said that he worked in China’s nuclear program and had access to the archive where classified documents were stored. He went there after hours one night, scooped up hundreds of documents and stuffed them into a duffel bag, which he then tossed out a second-story window to evade security guards. Unfortunately, the bag broke and the papers scattered.

Outside, he collected the files and stuffed them back into the torn bag. Although many of the documents were of interest for their intelligence content, it was the one about the W-88 that roiled American counterintelligence most because it contained highly classified details about a cutting-edge warhead design.

The United States had been producing small nuclear warheads for decades, and the Chinese were desperate to find out how to build miniaturized warheads themselves. China’s military was, and still is, playing catch-up to the United States.

China’s success in obtaining the secret design of the W-88 is the most dramatic example of a fact that United States counterintelligence agencies have been slow to recognize: just as China has become a global economic power, it has developed a world-class espionage service — one that rivals the C.I.A.

During the cold war, dozens of counterintelligence agents in the F.B.I. and the C.I.A. pursued Soviet and then Russian spies. The K.G.B. was seen as the enemy; China took a back seat. Only a handful of F.B.I. agents specialized in Chinese spy cases, and their work was not regarded as career-enhancing. Washington’s ongoing failure to make Chinese espionage a priority has allowed China to score a number of successes in its espionage efforts against the United States....