evidence: Manchuria was a vast area with a population of 40 million, the greatest concentration of heavy industry and railways in China, and enormous reserves of coal, iron, and many other minerals. The Soviet Union had promised the Nationalist government that it would withdraw its occupying armies within 90 days of Japan’s surrender and return the region to China. The government was determined to control Manchuria, which was vital to China’s future as a world power. However, Lin Biao’s army attempted to block the entry of Nationalist troops by destroying rail lines and seizing areas around ports of entry. Soon the two sides were locked in a fierce struggle for the corridors into Manchuria, although negotiations were under way in Chongqing between Mao Zedong and Chiang for a peaceful settlement. The Soviet army avoided direct involvement in the struggle, but it dismantled much industrial machinery and shipped it to the Soviet Union together with hundreds of thousands of Japanese prisoners of war. By the end of 1945 the Nationalists had positioned some of their best U.S.-trained armies in southern Manchuria as far north as Mukden (present-day Shenyang), a strategic rail center to which Nationalist troops were transported by air.