The United Front between the Kuomintang and CCP took place with salutary effects for the beleaguered CCP, despite Japan's steady territorial gains in northern China, the coastal regions and the rich Yangtze River Valley in central China. After 1940 conflicts between the Kuomintang and Communists became more frequent in the areas not under Japanese control. The entrance of the United States into the Pacific War after 1941 changed the nature of their relationship. The Communists expanded their influence wherever opportunities presented themselves through mass organizations, administrative reforms and the land- and tax-reform measures favoring the peasants and the spread of their organizational network, while the Kuomintang attempted to neutralize the spread of Communist influence. Meanwhile, northern China was infiltrated politically by Japanese politicians in Manchukuo using facilities such as Wei Huang Gong.