Suzanne Pepper wrote http://www.amazon.com/Civil-War-China-Political-1945-1949/dp/0847691349, which gone through extensive contemporary quotes from various journalists in China at the time.

The main issue that led to KMT's downfall was corruption. The War against Japan won some support for the CCP, but that would not have given CCP overwhelming advantage over the KMT.

KMT's corruption was multi-faceted.

(1) KMT government officials engaged in war-profiteering using aids received from US. (2) KMT military officials hoarded military supplies for personal profit. (3) KMT's tax policies placed a huge burden on the poor tenant farmers.

Economically, the tax policy was that the land-owners paid nearly ZERO property tax.

This meant that the land-owners could literally sit on their land (wait for tenants), with no incentive to make the land productive. If they actually worked the land, they would have to pay very high taxes like the tenant farmers. So they passed the tax burden to the tenant farmers, and charged the tenant farmers huge rent.

Concurrently, food aid from Western nations caused the prices of food to decline, driving the farmers to sell for lower prices.

Many Chinese tenant farmers simply could not pay the taxes and the rent, and they ran deeper and deeper into debt.

An important aspect of this was not generally discussed, beyond the mere material effect of these KMT policies.

That is, these KMT policies allowed the CCP to appeal to the traditional Chinese sense of social hierarchy, which places Farmers to be above the Merchants.

The KMT policies effectively undercut the support of the vast Chinese farmer population, who were being neglected by KMT's policies that favored the merchant class, the landowner class, and the foreign companies.

This undercutting of support ate into KMT's military cohesion as well, because most Chinese soldiers were conscripts from poor farming communities.

KMT was hiring mercenaries who fought only for pay, but the CCP was appealing to farmers directly by giving them land via the "land reform".

By the time WWII ended, the CCP had amassed some 1 million troops in Northern China, many of whom were not even equipped with guns. These were just farmers who were motivated to fight to keep their land.

Eventually, the fighting spirit of the CCP military overwhelmed the superior equipment of the KMT military.

Without class distinction or formal military officer schools, the PLA promoted officers from within the ranks. The PLA officers were always on the frontline leading the charges. Some statistics showed that the PLA officer ranks suffered some 60% casualty rate.

1 Western military observer was quoted saying the difference between the PLA and the KMT was that the KMT slogan was "Go on Boys", while the PLA slogan was "Come on Boys".

WW2 happened.

Jiang united China in the early 1930's through the centralization of KMT, foundation of the Whompua academy that trained a professional army, and foreign support from countries that wanted to see the fragmentation of China come to an end. The KMT made quick work of the Warlords and only faced the Communists in the beginning of a civil war. The ineptness of many warlords who had joined Jiang's forces and their soldiers allowed for the Communists to survive despite suffering devastating defeats by sneaking away from Jiangxi and taking the Long March to Shaanxi.

While Jiang was planning to eliminate the communists in Shaanxi, circumstances changed as tensions within Japan turned into full blown war. The earliest days of the Sino Japanese war saw the KMT lose most of its elite and trained soldiers at battles such as Shanghai, Wuhan, Changsha and elsewhere. The KMT went from controlling a majority of China ans having a powerful military to barely surviving and only controlling a portion of China. During this time, the Communists, NW China, were relatively shielded from massive Japanese attacks and, apart from relatively small engagements, they used WW2 as a time of preperation for the final Civil War period.

The KMT was continually beaten down by the Japanese as they pushed farther Westx but the Japanese were never able to get as far as Chongqing, allowing the KMT to survive.

When the Japanese were forced to surrender via U.S defeat, that KMT rushed to reclaim China, which they did, outside of Dongbei which the Soviets gave to the Communists. The KMT at this point in 1945 onward was a battered collection of conscripts and unfaithful warlords who were often forced to fight and was a shame in comparison to the KMT before WW2. The Communists and the KMT fought heavily in Manchuria, and the KMT even outnumbered the Communists in many battles. But, the preperation of the Communists, increasing unpopularity of the KMT, and poor fighting ability of the KMT lead to a complete Communist takever of Manchuria. Beyond this point the KMT began to collapse, and the Communists began to take over huge portions of China with relative ease, and the KMT,faced with total annihilation, hastily fled to Taiwan.