Nowhere is that dynamic more obvious than in the Taiwan Strait and the shipyards of southern Taiwan. On an island at the center of regional anxieties, Taiwan’s coast guard is expanding far more rapidly than its Navy while confronting almost daily challenges from China.

On one recent visit to an industrial area just outside the port of Kaohsiung, workers put the final touches on repairs for a coast guard patrol boat whose nose had been sheared off at sea.

“A Chinese ship hit this boat and broke right through it,” said Hu Yenlu, a former Taiwanese Navy officer who runs Karmin International, a company that builds and repairs Taiwanese Coast Guard vessels.

A few weeks earlier, he said, the patrol boat — a 36-foot rigid inflatable, similar to assault craft used by U.S. Navy Seals — had helped form a cordon with a few others around a suspicious-looking speedboat near Taiwan’s outer islands. That boat had six engines, a common design for China’s maritime militia, and when the Taiwanese Coast Guard asked about its mission, the pilot pushed the throttle and punched through.

“There was no name on that ship, but we know it was Chinese,” said Mr. Hu, recounting the story officials had told him. “When you don’t see a name, you know it’s suspicious.”