After achieving victory in the Battle of Shanghai by mid-November 1937, Japan's Army General Staff, which was in charge of military operations, adopted a policy of non-expansion of hostilities with the aim of ending the war. [This is presumably referring to the initiation of all out hostilities in the Battle of Shanghai.] On November 7 its de facto leader Deputy Chief of Staff Hayao Tada laid down an "operation restriction line" preventing its forces from leaving the vicinity of Shanghai, or more specifically from going west of the Chinese cites of Suzhou and Jiaxing. The city of Nanking is located further inland 300 kilometers west of Shanghai.
However, a major rift of opinion existed between the Japanese government and the two field armies it had deployed in Shanghai, the Shanghai Expeditionary Army (SEA) led by General Iwane Matsui and the 10th Army led by Lieutenant General Heisuke Yanagawa, both of which were nominally under the control of the Central China Area Army also led by Matsui. Matsui had made clear to his superiors even before he had left for Shanghai that he wanted to march on Nanking. He was convinced that the conquest of the Chinese capital city of Nanking would provoke the fall of the entire Nationalist Government of China and thus hand Japan a quick and complete victory in its war on China. Yanagawa was likewise eager to conquer Nanking and both men chafed under the operation restriction line that had been imposed on them by the Army General Staff.
On November 19 Yanagawa ordered his 10th Army to pursue retreating Chinese forces across the operation restriction line to Nanking, a flagrant act of insubordination. When Tada discovered this the next day he ordered Yanagawa to stop immediately, but was ignored. Matsui made some effort to restrain Yanagawa, but also told him that he could send some advance units beyond the line. In fact, Matsui was highly sympathetic with Yanagawa's actions and a few days later on November 22 Matsui issued an urgent telegram to the Army General Staff insisting that "To resolve this crisis in a prompt manner we need to take advantage of the enemy's present declining fortunes and conquer Nanking... By staying behind the operation restriction line at this point we are not only letting our chance to advance slip by, but it is also having the effect of encouraging the enemy to replenish their fighting strength and recover their fighting spirit and there is a risk that it will become harder to completely break their will to make war."
Meanwhile, as more and more Japanese units continued to slip past the operation restriction line, Tada was also coming under pressure from within the Army General Staff. Many of Tada's colleagues and subordinates, including the powerful Chief of the General Staff Operations Division Sadamu Shimomura, had come around to Matsui's viewpoint and wanted Tada to approve an attack on Nanking. On November 24 Tada finally relented and abolished the operation restriction line "owing to circumstances beyond our control", and then several days later he reluctantly approved the operation to capture Nanking. Tada flew to Shanghai in person on December 1 to deliver the order, though by then his own armies in the field were already well on their way to Nanking.