Complex sentence: Ma Buqing had earlier fought against the Japanese, but because the Soviet threat was great, Chiang changed Ma's position, in July, 1942 by instructing Ma to move 30,000 of his troops to the Tsaidam marsh in the Qaidam Basin of Qinghai.
Split into simpler sentences: Ma Buqing had earlier fought against the Japanese, but because the Soviet threat was great, Chiang made some arrangements regarding Ma's position. In July, 1942 Chiang Kai-shek instructed Ma Buqing to move 30,000 of his troops to the Tsaidam marsh in the Qaidam Basin of Qinghai.