How did the soviet union the chinese communists?

After the fall of the last Dynasty in 1911 there was a long period when various groups struggled for control. The Nationalists were generally recognized by most foreign governments as the "legitimate government of China" but they only controlled a small portion of the country. Most of it was broken up and ruled by local "War Lords" who would loosely ally themselves with the Nationalists and be recognized as "Governors" of the region they controlled.

The main rival to the Nationalists claim to power that could do anything about it were the Communists under Mao. The Communists and the Nationalist fought a protracted civil war before WW II, but called a truce to face the Japanese invasion in the 1930s. However, they did not prosecute that war very vigorously depending on the Americans to beat the Japanese and get them off their necks eventually. They both tended to squirrel away weapons so they could resume their civil war once Japan was out of the picture.

Before WW II the USSR would help first the Nationalists, then the Communists, whichever seemed to be in their best interest at the moment. They actually preferred China to be weak and divided because they were afraid a strong China might be a rival.

At the end of WW II Russia invaded Northern China and destroyed the Japanese Army deployed there. In the aftermath of WW II they backed the Communists in the renewed civil war and turned over large stockpiles of Japanese weapons they had captured during their invasion.

In the end the Communists won the civil war and the Nationalists retreated to the Island of Formosa (now known as Taiwan). So, in the end the Soviet Union did support the Communist victory in China. However, they were right...a strong China did emerge as a rival for leadership of the Communist World and demanding territory seized from China in the late 19th and through the mid-20th centuries be returned. The two nations went so far as to fight several boarder wars against each other in the mid to late 60s.