When young college students in Seoul went out to march through the streets calling for Park Geun-hye’s impeachment in a long streak of demonstrations that started last October, it wasn’t difficult to bump into an acquaintance blocking you — dressed in a navy military drab armed in a combat gear. These policemen were some of the most visible — and the most silent — presences, patrolling the massive candlelight vigils that eventually contributed to Park’s ouster from office.