TAIPEI — There’s a term in Taiwan for the hardened, military-aged Americans who mysteriously pop up at the island’s annual simulation of a Chinese invasion.

“English teachers.”

Vaguely employed and surprisingly fit, they are often spotted in small groups, chatting with local soldiers, walking double-time on the streets, and giving death stares to any camera-toting reporters as the Taiwanese prepare and execute their live drills.

Local media estimated that over 100 of these foreigners — rumored to be unofficial US military advisors and observers — were present at this year’s Han Kuang exercise, which featured 10 intense days of drills across…