 Complete the following phrase:  American military forces gradually solidified this defensive position on the southeast side of the Korean peninsula, dubbed the Pusan Perimeter. Walker received reinforcements, including the Provisional Marine Brigade, which he used along with the Armys 27th Infantry Regiment as fire brigades, reliable troops who specialized in counterattacking and wiping out enemy penetrations. As more reinforcements arrived, the combat advantage shifted toward the United Nations forces. North Korean forces had suffered terribly and their supply lines were under constant aerial bombardment. Almost all of their Russian-made T-34 tanks, which had spearheaded the invasion, had been destroyed. Walker ordered local counterattacks, while planning for a large-scale breakout in conjunction with MacArthurs 