A small island controlled by Taiwan a few miles off China’s coast lived for decades in constant readiness for war. At one point in 1958, troops there hunkered in bunkers as Communist forces rained hundreds of thousands of shells on them.

These days, the island, Kinmen, has become a hub of Taiwan’s commerce with China and its abandoned, weatherworn fortifications are tourist sites. Eight ferries a day take Taiwanese businesspeople and visitors from Kinmen to mainland China.

But the sea around Kinmen has again turned tense after two Chinese men onboard a speedboat died in the area last month while trying to flee a Taiwanese Coast Guard vessel.

The Chinese Coast Guard has responded by patrolling close to the island, and briefly boarded a Taiwanese tourist boat last month. In mid-March, four boats came as close as 3.5 miles off Kinmen’s shore, entering what Taiwan calls a prohibited zone.

China has said the patrols are to protect Chinese fishing boats. But the patrols also fit more broadly with China’s strategy of squeezing Taiwan, an island-democracy that Beijing claims as its territory, while stopping short of setting off a major confrontation that would draw in the United States.