By the time 3 RAR arrived in Pusan on 28 September , the North Koreans were in retreat following the Inchon landings . As a part of the invasion force under the UN Supreme Commander , General Douglas MacArthur , the battalion moved north and was involved in its first major action at Battle of Yongju near Pyongyang on 22 October , before advancing towards the Yalu River . Further successful actions followed at Kujin on 25 – 26 October 1950 and at Chongju on 29 October 1950 . North Korean casualties were heavy , while Australian losses included their commanding officer , Lieutenant Colonel Charles Green , who was wounded in the stomach by artillery fire after the battle and succumbed to his wounds and died two days later on 1 November . Meanwhile , during the last weeks of October the Chinese had moved 18 divisions of the People 's Volunteer Army across the Yalu River to reinforce the remnants of the KPA . Undetected by US and South Korean intelligence , the 13th Army Group crossed the border on 16 October and penetrated up to 100 kilometres ( 62 mi ) into North Korea , and were reinforced in early November by 12 divisions from the 9th Army Group ; in total 30 divisions composed of 380 @,@ 000 men . 3 RAR fought its first action against the Chinese at Pakchon on 5 November . The fighting cost the battalion heavily and despite halting a Chinese division the new battalion commander was dismissed in the wake . Following the Chinese intervention , the UN forces were defeated in successive battles and 3 RAR was forced to withdraw to the 38th parallel .