By early August 1945, only one major Sino-Soviet issue was left unresolved, a detailed agreement as to where exactly the Sino-Soviet boundary ran. With Japan's surrender imminent, it was important to Chiang Kai-shek that the agreement on Outer Mongolia be announced soon, so that the Chinese people could be convinced that Outer Mongolia's loss was a necessity of war. On 11 July 1945, Soong expressed his concern that the joint Sino-Soviet declaration on Outer Mongolia not mention the border, since there were still many disagreements about the disposition of the boundary line. Stalin agreed, suggesting that they retain the “status quo,” but Soong retorted: “These is dispute about status quo.”1 Negotiations were broken off after 12 July, so that Stalin and Molotov could attend the conference at Potsdam, but talks resumed once again on 7 August. Coincidentally, during this almost four-week break, the Chinese envoy T. V. Soong had the opportunity to travel back to Chungking to consult personally with his brother-in-law Chiang Kai-shek. Based on Chiang's decisions, the final points of the upcoming Sino-Soviet declaration were determined between 7 and 14 August 1945.

The issue of Outer Mongolia's borders with China was of great importance to Soong. After returning from Chungking, he presented two maps to Stalin, one Russian and one Chinese, which outlined the Sino-Mongolian border. The following exchange then took place:

But, on 10 August, Stalin disputed the maps which Soong had presented: “Re frontiers Chinese map is not well founded. Existing frontiers should be recognized.”2

Stalin stuck to this proposal because the existing borders actually included extensive Manchurian territories which Tokyo had secretly ceded to Moscow during Outer Mongolia-Manchukuo border negotiations in the 1930s. Stalin clearly hoped to retain the largest amount of Chinese territory possible. The Chinese delegation, on the other hand, pleaded with Stalin to define the border between Outer Mongolia and China, even suggesting that the two countries use a Chinese college atlas as their guide.