South Korean police on Monday tried for hours to block a civic group from sending pro-democracy leaflets across the border by balloon after North Korea threatened the activists with a “merciless military strike,” but the South’s efforts ultimately failed.

Hundreds of police swarmed roads near the peninsula’s demilitarized zone, encircling the vehicles that were transporting activists to the planned launch site. After a 3 1/2 hour standoff, the activists retreated, 200,000 leaflets still in the back of a van.

But hours later, after backtracking and eluding police, about 10 activists — compared with the 80 who started the day — made it to Ganghwa Island, not far from the border, where they released half of their leaflets.