At 21 : 00 the first shells of what proved to be a two @-@ hour North Korean artillery and mortar preparation against the American river positions of 2nd Platoon . As the barrage rolled on , North Korean infantry crossed the river and climbed the hills in the darkness under cover of its fire . At 23 : 00 the barrage lifted and the North Koreans attacked 2nd Platoon , forcing it from the hill after a short fight . Similar assaults took place elsewhere along the battalion outpost line .