At the start of the Civil War in 1927, the KP always had the upper hand. They had control of the main business and economic centres in China (Eastern Seaboard) and had support of the wealthier landowners and middle classes. They had all the money and equipment as well the better trained army. Unfortunately, they did not have the support of the common peasants; which the CCP did. In 1934, the CCP went on the Long March; which enabled the Communist message to be spread amongst many peasants. The Nationalists did attack the communists on many occasions but the CCP marched on.
In 1936, Japan took advantage of the Civil War and invaded more parts of China (as it already controlled Manchuria). Chiang ignored this threat, saying that the Japanese were a disease of the skin; which could be cured. Communism is a disease of the soul that affects the whole body. Chiang was obviously more interested in defeating the Communists than actually fighting foreign invaders. Chiang was forced to side with his enemies but the Japanese simply swept him and his armies into the mountains of Sichuan.
Chiang, in a moment of supposed “intelligence” destroyed his own people’s crops so that the Japanese could not have them. This did absolutely nothing to the Japanese, as they brought all supplies in by air or sea, and only infuriated his people even more. Chiang, with the outbreak of the WWII, began to get support from the Allies. Stupidly, Chiang did not use these supplies on the Japanese; instead storing them to use against the Communists. Those living in Nationalist areas were treated poorly: discipline was weak in the Koumintang as Chiang was proud and stubborn, letting his officers do what they want. After WWII, the Civil War was restarted. Chiang was confident as he had more men and weapons than Mao. Nevertheless, Mao used the same tactics on the KP as he did on the Japanese. The KP lost the support of the middle classes as they were seen as landlords and corrupt: this led to many officers defecting to the CCP. These were the main weaknesses of the Nationalists and ultimately led to their defeat. Chiang then fled to Formosa with a lot of gold and a core of advisors.
The Communists however, overcame their weaknesses, allowing them to be victorious over the KP. They walked for over 1000 km and lost 80% of their men; but still they walked. Whilst walking they spread the Communist message to peasants, who saw any other option than the Nationalists as a good one. The Communists treated the peasants nicely whilst staying with them, unlike the Nationalists.
Whilst fighting the Japanese, the communists managed to overcome their weaknesses. They used guerrilla tactics to gain weapons and vehicles’ to great affect and soon had control over a lot of the land. They cut communication, blew up vital transport links and ambushed Japanese troops. They taught many civilians to spy on the Japanese whilst doing the most basic household tasks. When the Japanese were spotted everything was buried or hidden and items were booby-trapped; so much so that towards the end of the war, Japanese troops dared not enter a deserted village. Along side self defence, peasants were taught Communist beliefs.
After the Japanese pulled out, Mao employed the same tactics on the KP that were used on the Japanese and a lot of Nationalists were killed. More and more equipment was being seized from the KP and often just given: many soldiers, fed up with the Nationalist regime, defected; bringing with them ammunition, weapons and vehicles. The CCP renamed itself – from the Red Army to the People’s Liberation Army – and doubled in size. They were able to fight battles in open ground: without the need of any ambush. In 1949, Shanghai fell and China was declared a Communist Republic.
But the main reason that the CCP won as because it had a superb, tactically minded leader – Mao. It was he who continued to march the Long March; though he lost over 80% of his men. It was he instructed his men to be kind to the peasants and fight the way they did. It was he who persevered against both his enemies and fought against condemnation from the wider world. It was HE who over came his weaknesses by gaining equipment and support in the ways mentioned above. This in turn is likely to have led to his victory over the Nationalists. Chiang on the other hand did not overcome his weakness. He let discipline fail and treated people badly. Therefore, he lost support and men (via. defection) to the CCP. This in turn, probably, caused him to lose the war and flee to Formosa.
So in conclusion, it was the leadership which decided who won the civil war. It was the great leadership of Mao that made him enforce strict rules on his men; leading to his popular support. He could then stay with peasants, enabling him to ambush his enemies.
Chiang, on the other hand, did not enforce discipline and so many people dislike him; including his soldiers, with many defecting. Leadership is why the Communists won the Civil War.