Second Sino-Japanese War , —45 , conflict that broke out when China began a full-scale resistance to the expansion of Japanese influence in its territory which had begun in For much of the early 20th century, Japan had exercised effective control of Manchuria , initially through the terms of the Twenty-one Demands and later through its support of Chinese warlord Zhang Zuolin. However, a serious conflict was developing, and the Chinese in Manchuria were especially restive under the privileges held by the Japanese. Chinese citizens formed the vast majority of the population, and the legal title of the region was held by China. Yet Japan controlled much of south Manchuria through its railways and its leasehold on the Liaodong Peninsula and in other ways that compromised Chinese sovereignty. In an attempt to assert their independence, the Chinese began building a series of railroads that would in part encircle the Japanese lines and terminate at Huludao , a port which the Chinese were developing.