The Second Sino-Japanese War was not just a war between Japan and China, but involved many nations that had different vested interests that influenced their positions and actions taken during different phases of this war. It is clear that China had an intensely difficult task at hand in attempting to win Allies' support while they had motives not necessarily in congruence with China's.
Therefore in order to understand the complexity of the involvement of China and Japan, and the later involvement of the Soviet Union, the United Kingdom, the United States and France in the Second Sino-Japanese War, it is important to appreciate the underlying reasons and motives of the different parties that they brought to the war.
Empire of Japan: Imperial Japan launched the war in an effort to destroy the Chinese central government under the Kuomintang, and to create puppet governments that followed Japanese interests. From these actions Japan would obtain a secured supply of raw materials and a market to ensure the prosperity of the Japanese home islands. However, Japan's inability to bring the war in China to an acceptable conclusion, coupled with increasingly unfavorable trading restrictions from the West in response to Japan's continued actions in China, meant that Japan also needed to control sizable natural resources, such as those in Malaysia, Indonesia, and the Philippines, which were at the time controlled by Britain, the Netherlands, and the USA, respectively. Japan's strategy to seize these embargoed resources led to the attack on Pearl Harbor, opening the Pacific Theater of World War II.
China (Kuomintang): Before the onset of full scale war, Nationalist China focused its energies on modernizing the army and building a viable defense industry to increase its combat power vis-à-vis Japan. Because China under the Kuomintang was unified only nominally, it was also constantly preoccupied with fighting internal wars against the communists, resurgent warlords, and other militarist factions. However, once the war against Japan broke out, backing down was impossible, even though China was far from prepared for fighting a war on such a massive scale against a vastly superior enemy. In sum, Nationalist China had several goals: to resist Japanese aggression, to unite China under one central government, to rid China of foreign imperialism, to defeat communism, and to re-emerge as a strong country. In essence, the war of resistance was seen by many as a war of national revival.
China (Communist): Chinese Communists generally avoided large-scale frontal fighting against the Japanese, while conducting guerrilla warfare and political activities in occupied territories to expand their base areas. As one of its main goals was expansion, the CCP sought to avoid direct conflicts with the Japanese Army in order to emerge from the war stronger than the Nationalist forces, so in the inevitable struggle for dominance, the CCP would be the victor.
Soviet Union: To allow Japan to overextend itself in China such that the USSR could fight Germany in the West without having to garrison strong forces in the East against possible Japanese aggression. Also, a weakened China would allow Chinese Communists to develop and eventually take over the country, providing a potential ally and a buffer zone against Western and Japanese expansionism.
United Kingdom: Throughout the 1920s and 1930s, British attitude was conciliatory toward Japan, as the two had already formed the Anglo-Japanese Alliance. Many in the British community in China also supported Japanese actions to weaken the Chinese Nationalist government. This was because British economic interests suffered substantially when the Chinese Nationalist government successfully revoked much foreign concessions, and regained the right to set its own tariff without British influence. Once World War II began, the UK had to fight Germany in Europe while at the same time hoping China and Japan fight to a stalemate, in order to buy time to regain its Pacific colonies in Hong Kong, Malaysia, Burma, and Singapore. The majority of British forces were committed to fighting in Europe, and could spare little for the war in the Pacific.
United States: The United States was generally isolationist prior to the attack of Pearl Harbor and did not wish to directly provoke Japan, while it aided China with its volunteer airmen and oil/steel embargoes. Following the American entrance to World War II, the US had to defeat Japan in the Pacific while also fighting Germany in the European Theater, with the emphasis on defeating Germany first. The US began a campaign of island hopping in order to secure bases close enough to Japan to support bombing raids and an eventual invasion. When Germany capitulated, the war in the East was to be finished as fast as possible with minimal US casualties. President Franklin Roosevelt also initially wished to aid China so that it would emerge as a democratic nation friendly to the US and as a large market for US trade. Later however, he began to view Chiang Kai-shek as a threat the would rival US interests in East Asia much like Japan had done prior to the war.
Vichy France: With massive U.S. supply coming to Yunnan through French Indochina's northern state of Tonkin (northern Vietnam), the Japanese wanted to blockade the Chinese-Indochinese border. In 1940, following the establishment of the Vichy France regime, Japan staged an invasion of French Indochina. In March 1945, the Japanese staged a coup d'état in French Indochina and created their own colonies as the Empire of Vietnam.
Free France: In December 1941, following the Japanese attack on Pearl Harbor, Free France leader Charles de Gaulle declared war on Japan. In 1943 de Gaulle created the Light Corps of Intervention (CLI) Special Forces within the FEFEO to support the French Resistance in Indochina. From 1944 to 1945 the CLI operated against the Japanese in Vietnam, using the southern Chinese border as a sanctuary. In September French general Leclerc heading the CEFEO signed the armistice with Japan and landed in Vietnam and Java with the Franco-British task force to regain control of the colony in October. A partition was established with the Communist Chinese controlling north Vietnam and the British-French controlling southern Vietnam until 1946 when the First Indochina War broke out.