The only thing I know was that when the Soviet Union intervened in 1945, they captured Manchuria from the Japanese and gave it to the Communists, allowing them to have a big power base in the area (And of course, North Korea was established after WW2, who helped the Chinese with volunteers and material).
I know that the Japanese fought against both the Republicans and the Communists, but from what I can tell, the Japanese mostly fought the former.
To put it bluntly, it effectively reversed the power positions of the Nationalists (Kuomintang, or KMT) and the Communists (CPC). Prior to the Second Sino-Japanese War, the Nationalists controlled most of the country, had a nascent industrial base budding in Jiangsu, and westernized military forces. Meanwhile, the Communists had been betrayed, severely weakened, and were on the run.
Over the course of the Second Sino-Japanese War, the Nationalists were badly battered. To begin with, they lost their capital of Nanjing, their industrial and political base province of Jiangsu, their vaunted German-trained divisions, and nearly all their armor and aircraft in the opening stages of the war. Then they had to move their capital repeatedly after it fell to the Japanese again and again. They lost significant amounts of manpower and resources because as the "ruling party" of China, they were expected to fight on the frontlines. Meanwhile, the Nationalists and Communists signed an uneasy armistice to temporarily stop fighting with one another and unite against the Japanese (albeit grudgingly on the Nationalist leader, Chiang Kai-shek's part, who had to be kidnapped by his own men before he ceded).
While the Nationalists fought on the frontlines, however, the Communists engaged in a guerilla war against the Japanese, conserving their resources and swelling their manpower with popular public support. The public was also disillusioned with the "democratic dream of China", due to repeated blows to the ideology since the machinations of Yuan Shikai and the disastrous Warlord Era that followed.
By the time the Second Sino-Japanese War ended in 1945, the Nationalists had lost a significant part of their strength, while the Communists were resurgent and stronger than ever, controlling 1/3rd of the Chinese population and 1/4th of Chinese territory when previously they had little to nothing. In terms of ideology, much of the population was now leaning towards the Communists. Both sides immediately re-engaged hostilities, but at this point the weakened Nationalists could not fend off the Communists, and the Communists emerged victorious, occupying mainland China while the Nationalists fled to the island of Taiwan - where they remain in 'exile' to this day.
References:
Wilson, Dick. When Tigers Fight: The Story of the Sino-Japanese War, 1937-1945. Penguin Group USA, 1982
Fenby, Jonathan. Generalissimo: Chiang Kai-shek and the China he lost. Simon and Schuster, 2003.
Hsu, Immanuel Chung-yueh. "Rise of modern China." (1975).