Snipers in the guard towers would fire at them. North Korean patrols would shoot at them from across the MDL. The North Koreans would even cross the MDL to assault a patrol. They then hightailed it back across to avoid return fire. The rules of engagement allowed Dennis to return fire inside the DMZ. He had to cease fire once they crossed back over the MDL. There were provisions that did allow engaging combatants across the border. All that was needed was permission from higher up. By the time it took to get permission, it was too late. The North Koreans knew the firing rules that patrols had to follow. They would use them against our forces every opportunity they could.
Therein lies the dilemma in North Korea. US and ROK soldiers had to follow strict rules against an enemy that ignored the rules. A patrol comes under fire and takes casualties. Several dead and several wounded friends. The cowardly North Koreans that did this simply put their tail between their legs and ran back across the border. It was over. No retaliation. No pursuit. The Armistice Agreement says neither side could cross the MDL. They did it anyway. All the time. With no consequences. Sure, the UNC would call a MAC meeting. Both sides would travel to the JSA and gather at the peace table. Complaints would be made and the North Koreans would deny it. Four to five times a month these meetings were called. North Korea would be accused and they would deny it. It was like a never-ending movie from hell. The storyline changed every day but the ending was always the same. This is exactly why the North Koreans continue to do what they want 60-plus years later. They're never held accountable.