After the beginning of the Sino-Soviet split, Mongolia briefly vacillated, but soon took a sharply pro-Soviet stand, being one of the first socialist countries to endorse the Soviet position in the quarrel with China. Military build-up on the Sino-Mongolian border began as early as 1963; in December 1965 the Mongolian Politburo requested the Soviet Union to station its military forces in Mongolia. In January 1966, with Leonid Brezhnev's visit to Mongolia, the two countries signed a mutual assistance treaty, paving way to Soviet military presence in the MPR. In February 1967, following weeks of worsening Sino-Soviet tensions, Moscow officially approved the stationing of the reorganised 39th Soviet Army in Mongolia.