What is the strategic policy of marching north and defending south? What is its history?

In order to defend the victory of the people's War of Resistance against Japanese Aggression and strengthen the people's revolutionary forces, on September 1945, the Central Committee issued instructions to all C.O., clearly stipulating that "developing north and defending south" is the party's "national strategic policy". Its main contents are: making concessions in the south and shrinking the southern defense line; Consolidate the liberated areas in North China, East China and Central China; Control Jehol and Chahar provinces and concentrate on controlling the strategically important northeast region. According to the strategic deployment of the central government, a large number of main forces quickly went north to try to control the opportunity of hot inspection and entering the northeast, which broke the Kuomintang's attempt to monopolize the northeast and attack the liberated areas in Shanhaiguan pass. At the same time, this strategic policy made the troops scattered in the south concentrate in Jiangbei in time, avoiding the danger of being split in two, and enhancing the strength of the liberated areas in East China and North China. In this way, a favorable strategic situation has been formed to counter the Kuomintang military attack. Practice has proved that "developing northward and defending southward" is a far-sighted strategic decision made by the CPC Central Committee, which is of great and far-reaching significance for winning the war of liberation. I hope this answer is helpful to you.