Final answer:
Joseph Stalin observed the actions of the Western powers as Japan became an aggressor. He recognized the need to protect the Soviet Union and came to an agreement with Hitler, while keeping an eye on global developments.
Step-by-step explanation:
The European leader who sat back and observed the actions of the Western powers as Japan became an aggressor in the Far East was Joseph Stalin.
Step-by-step explanation: While the Western powers were concerned with the threats posed by both Germany and the fascist nations, Joseph Stalin of the Soviet Union carefully watched the developments. Stalin was well aware of Hitler's ambitions and the geopolitical changes occurring. Despite opposing Hitler, Stalin recognized that the best way to protect the Soviet Union and potentially gain additional territory was to seek some form of agreement with Germany. This was cemented in the notorious Molotov-Ribbentrop Pact of August 1939, which dictated the division of Poland and non-aggression between the two powers. In this way, Stalin sought to ensure the Soviet Union's security, at least in the short term, as well as to monitor and adjust his strategy based on the actions of Japan, the European nations, and the emerging policies of the United States.