看看日本当年发布的宣战诏书。那里面列举的主要开战理由只有一个，那就是美国支持中国抗战，逼日本撤出中国。

To insure the stability of East Asia and to contribute to world peace is the far-sighted policy which was formulated by our Great illustrious Imperial Grandsire and Our Great Imperial sire succeeding Him, and which We lay constantly to heart. To cultivate friendship among nations and to enjoy prosperity in common with all nations has always been the guiding principle of Our Empire's foreign policy. It has been truly unavoidable and far from Our wishes that Our Empire has now been brought to cross swords with America and Britain. More than four years have passed since China, failing to comprehend the true intentions of Our Empire, and recklessly courting trouble, disturbed the peace of East Asia and compelled Our Empire to take up arms. Although there has been re-established the National Government of China, with which Japan has effected neighborly intercourse and cooperation, the regime which has survived at Chungking, relying upon American and British protection, still continues its fratricidal opposition. Eager for the realization of their inordinate ambition to dominate the Orient, both America and Britain, giving support to the Chungking regime, have aggravated the disturbances in East Asia. Moreover, these two Powers, inducing other countries to follow suit, increased military preparations on all sides of Our Empire to challenge us. They have obstructed by every means our peaceful commerce, and finally resorted to a direct severance of economic relations, menacing gravely the existence of Our Empire. Patiently have We waited and long have We endured, in the hope that Our Government might retrieve the situation in peace. But our adversaries, showing not the least spirit of conciliation, have unduly delayed a settlement; and in the meantime, they have intensified the economic and political pressure to compel thereby Our Empire to submission. This trend of affairs would, if left unchecked, not only nullify Our Empire's efforts of many years for the sake of the stabilization of East Asia, but also endanger the very existence of Our nation. The situation being such as it is, Our Empire for its existence and self-defense has no other recourse but to appeal to arms and to crush every obstacle in its path.