Hsu Keng-Jui has a half-smoked cigarette protruding from the corner of his mouth. He is a member of a volunteer network, most of whom are also veterans, that keeps tabs on the Chinese ships and planes that are currently always present just beyond Taiwan’s borders. Mr. Hsu attaches a long radio antenna to a steel railing with plastic zip ties, takes a seat with his portable radios, and starts listening to the military channels. The Taiwan coastguard guiding maritime traffic is the only sound we initially hear—a gentle southern lilt. Then, through the thick static, a different tone and accent emerge. It’s the Navy of China. Ahead of a crucial presidential election in Taiwan, an island it has long considered its own, China has been increasing the pressure.