American naval vessels routinely patrol the South China Sea. It is a shared maritime space, but China claims much of the area as its own. That much is true. What if one of the ships was torpedoed? The retired admiral James Stavridis teamed up with Elliot Ackerman, a journalist and former Marine, to write about how, in the shadow of an increasingly tense relationship between the U.S. and China, such an incident could spiral into catastrophe. The result is “2034: A Novel of the Next World War.” The book is a thriller, and also a cautionary tale; Stavridis cites Nevil Shute’s post-apocalyptic novel “On the Beach” as an inspiration. The writers tell Evan Osnos that they intend to deliver in fiction an ingredient that’s missing in military planning: “We have plenty of intelligence, we have plenty of hardware,” Ackerman notes, but “what we often lack is imagination.”