The lull in military offensive activity during the mid-1951 truce talks presaged the kind of warfare that would soon typify the final phase of the Korean conflict. Before the fighting settled into positional trench warfare reminiscent of World War I, the Marines participated in the final UN offensive. In a bitter struggle, the division hacked its way northward through, over, and around the Punchbowl, and in September 1951 occupied a series of commanding terrain positions that became part of the MINNESOTA Line, the Eighth Army main defensive line. Beginning on the 20th of that month, it became the primary mission of frontline units to organize, construct, and defend positions they held on MINNESOTA. To show good faith at the peace table, the UNC outlawed large-scale attacks against the enemy. Intent upon not appearing the aggressor and determined to keep the door open for future truce negotiations, the United Nations Command in late 1951 decreed a new military policy of limited offensives and an aggressive defense of its line. This change in Allied strategy, due to politico-military considerations, from a moving battle situation to stabilized warfare would affect both the tactics and future of the Korean War. Even as Allied major tactical offensive operations and the era of fire and maneuver in Korea was passing into oblivion, several innovations were coming into use. One was the Marine Corps employment of helicopters. First used for evacuation of casualties from Pusan in August 1950, the versatile aircraft had also been adopted by the Marine brigade commander, General Craig, as an airborne jeep. On 13 September 1951, Marines made a significant contribution to the military profession when they introduced helicopters for large-scale resupply combat operations. This mission was followed one week later by the first use of helicopters for a combat zone troop lift. These revolutionary air tactics were contemporary with two new Marine Corps developments in ground equipment--body armor and insulated combat boots, which underwent extensive combat testing that summer and fall. The latter were to be especially welcomed for field use during the 1951–1952 winter. Along the MINNESOTA Line, neither the freezing cold of a Korean winter nor blazing summer heat altered the daily routine. Ground defense operations consisted of dispatching patrols and raiding parties, laying ambushes, and improving the physical defenses. The enemy seemed reluctant to engage UN forces, and on one occasion to draw him into the open, EUSAK ordered Operation CLAM-UP across the entire UN front, beginning 10 February. Under cover of darkness, reserve battalions moved forward; then, during daylight, they pulled back, simulating a withdrawal of the main defenses. At the same time, frontline troops had explicit orders not to fire or even show themselves.[12] [12] Col Franklin B. Nihart comments on draft MS, Sep 66, hereafter _Nihart comments_. [Illustration: