The ramming of a Filipino fishing boat by a Chinese vessel last week marked an escalation in the South China Sea. But you wouldn’t know it listening to Philippine President Rodrigo Duterte, who has become exhibit A for the folly of appeasing Beijing’s geopolitical ambitions.

A Chinese fishing boat struck the F/B Gimver 1 on the edge of the Spratly Islands, which Beijing claims as its territory, sinking the ship and leaving 22 Filipino fishermen stranded at sea before they were picked up by a Vietnamese vessel. The Chinese...