There are three main reasons why the Second Sino-Japanese War was and is considered separate from the wider global war.

Eurocentrism is of course a major reason why so little attention has been paid to the Second Sino-Japanese War. Western attitudes towards the war both during and after had a tendency to consider it a side conflict, which is to some extent correct. The war from 1937-1941 was exclusively a Sino-Japanese conflict, and did not draw other regional powers (i.e. America, Great Britain and USSR) into a general war. The Japanese-Soviet clashes in 1939 had little connection to the Sino-Japanese War. On the other hand, the invasion of Poland in 1939 did lead to the involvement of Great Britain, France and a growing number of nations in a general war in Europe.

Even within China itself, the war was not linked with WWII. Up until the 1980s, the Second Sino-Japanese War was the 'War of Resistance', the epic struggle of Chinese resistance against Japanese aggression. Education textbooks published with the approval of the Chinese Communist Party had justifiable motivation not to link the war with the wider Pacific War, especially since 'capitalist imperialist America' played a bigger role in the final defeat of Japan than China did. There was little inclination to link the war with WWII since it would only downplay China's self-perceived contributions to the defeat of Japan. However, in the 2000s, there has been a strong push by the Chinese government to link the Sino-Japanese War with the Pacific War. This is due to China's growing stature as a superpower and its desire to have a greater say in international politics. The Pacific War is a good example of China collaborating with other major powers against a common enemy, and therefore has been used to illustrate that China was and is willing contributor to international society.

Historiography has also played a role in splitting the Sino-Japanese War from WWII. Much of the historiography on the Sino-Japanese War gives little thought to the implications of the wider Pacific War on the Chinese front and vice versa. The war can be easily portrayed as a pure Sino-Japanese conflict throughout 1937-1945 with minimal foreign involvement, and has been treated as such by many historians. It is only in the 2000s and 2010s that both Cambridge and Oxford, under professors Hans van de Ven and Rana Mitter respectively, have established projects that look at the Second Sino-Japanese War from a global context. The links between the war in China and the global war is still in its infancy in English-based historiography.