According to Chang and Halliday, under Mao's leadership CPC officials arranged ceasefires with the Japanese in central areas to protect Japanese train lines and allow time for an increase in CPC membership, all while claiming to be fighting the Japanese, and that as of late 1940, Mao was so focused on opposition to the KMT that he confided to top CPC officials that he wished for continued Japanese occupation of China.  Mao further consolidated power over the Communist Party in 1942 by launching the Zheng Feng, or "Rectification" campaign against rival CPC members such as Wang Ming, Wang Shiwei, and Ding Ling. Also while in Yan'an, Mao divorced He Zizhen and married the actress Lan Ping, who would become known as Jiang Qing.
During the Sino-Japanese War, Mao Zedong's strategies were opposed by both Chiang Kai-shek and the United States. The US regarded Chiang as an important ally, able to help shorten the war by engaging the Japanese occupiers in China. Chiang, in contrast, sought to build the ROC army for the certain conflict with Mao's communist forces after the end of World War II. This fact was not understood well in the US, and precious lend-lease armaments continued to be allocated to the Kuomintang. In turn, Mao spent part of the war (as to whether it was most or only a little is disputed) fighting the Kuomintang for control of certain parts of China. Both the Communists and Nationalists have been criticised for fighting amongst themselves rather than allying against the Japanese Imperial Army. However the Nationalists were better equipped and did most of the fighting against the Japanese army in China.
Willy Lam stated that during the war with Japan:
The great majority of casualties sustained by Chinese soldiers were borne by KMT, not Communist divisions. Mao and other guerrilla leaders decided at the time to conserve their strength for the "larger struggle" of taking over all of China once the Japanese Imperial Army was decimated by the U.S.-led Allied Forces.
My take is that there is strong suspicion that Mao's Red Army colluded with Japan that resulted them winning the civil war.
Chinese communists had top agents working inside of Japanese military and puppet Nanking government. Pan Hannian the top agent had an office in Shanghai under Japanese protection. Pan Hannian moved communist representatives out of HK through Japanese liaison. There is no dispute about the collusion.
Both Wang Ming and Vladimirov had observations of Chinese communist contacts with Japanese and puppet Nanking regime.
In any case, Japanese never treated Chinese communists seriously, knowing that Chinese communists never intended to make enemies with Japanese. Japanese had an understanding with communists in that the major railways like Tientsin-Pukow Railway should never be disrupted.
Mao's collusion with Japanese was something Stalin did not appreciate either. Mao was very much written off by Stalin during the war.
Chiang Kai-shek did not treat the communists as a serious risk either. Because Chiang knew it was a bluff that communists ever possessed 400,000-500,000 troops.
Vladimirov Diaries repeatedly pointed out that communists had a little above 300,000 troops.
The number is completely corroboarated by headcounts under each and every communist general during the initial phase of the civil war, from Aug to Oct 1945. I counted it one by and one.
I pointed out that Mao Tse-tung himself had betrayed the actual strength of communist forces in first demanding various units of 8RA divisions and columns as well as detachments to supply 150,000 "refined" troops for attacking wartime interim capital Chungking and subsequently lowered the par to 70,000 troops in late 1940.For New 4th Corps [New 4th Army], Mao Tse-tung was demanding N4C to supply 20,000 "refined" troops. Actual N4C troops numbered probably no more than 30,000 in total
- consisting of Chen Yi and Su Yu's 8000 troops [i.e., First Detachment, Second Detachment and Third Detachment] that routed Haan Deqing's Jiangsu troops at the Huangqiao Battle, less than 9,000 combat + 2,000 noncombat at N4C headquarters, and unspecified number under Fourth Detachment and Fifth Detachment as well as under Li Xiannian's guerrillas in Hubei Province. )
What's clear is that the above numbers were the apex of acclaimed communist troops. The actual numbers were much lower. For example, Peng Xuefeng's N4C Sixth Detachment, converted to 8RA Fourth Column, had 5000 guns and less than 10,000 men after three setbacks. After three major setbacks, Peng Xuefeng evacuated to Anhui-Jiangsu border, and would not return till three years later, only to get killed in civil war under a slogan that it was not too late for a gentleman to take revenge [against government troops] in three years [by taking advantage of Japanese Ichigo Campaign].
If you want to know what communists did before the Wan-nan Incident, http://www.republica...an-Incident.pdf
then look for the section on http://www.republica...FROM-WITHIN.pdf
After the German attack at Soviet Union, Mao revised his policy to show some restraint in attacking government troops. In any case, communists had ceased its war against Japanese, after the Hundred Regiment Campaign. The only reason that communist forces had to fight a railway disruption war was Japanese success in paving railways and highways across North China after communist forces routed all government troops and government guerrillas across Hebei-Shandong-Shanxi provinces. Per Xie Youtian research, the actual communist strike regiments exerted to the campaign would total about 22 regiments.
(Per Xie Youtian research, the damages assessed by Japanese were far below what communist propaganda claimed: 48 bridges blew up on Zheng-tai Railway, 7 train stations, 40 points of sabotage, 2 water towers, 7 tunnels; Ping-han Railway suffered 19 bridges, 67 spots, 12 stations, 38km phone line; and Tong-pu Railway had destruction of 6 bridges, 7 spots, one station, 3 water towers. )
The civil war could very well be reversed should Chiang Kai-shek be able to take preemptive strike, including partial peace with Japan, to segregate the Chinese communists from the Russians.