By the end of August 1950, the initial North Korean onslaught was reversed and Seoul was retaken. As the United Nations forces advanced into North Korea, forces from the Communist China stepped in to help their North Korean allies. The UN advance ground to a halt in December, then retreated south in early 1951 while tactical aircraft continued to support of United Nations forces. Eventually, the line stabilized along the 38th Parallel, where a stalemate ensued for the next two years.