What major theme of World War II involved the Allied Powers fighting against territorial aggression?

The major theme of World War II that centered on the Allied Powers fighting against territorial aggression is closely related to the concept of Axis Powers' Expansionism. During the interwar period, Germany, Italy, and Japan, collectively known as the Axis Powers, pursued aggressive expansionist policies, seeking to enlarge their territories through military conquest.

The invasion of Poland by Germany in 1939, followed by further territorial acquisitions in Europe and Asia, exemplifies this aggression. The Allies, recognizing the threat posed by this expansionism to global stability and the sovereignty of nations, banded together to combat and eventually halt these aggressive actions. The conflict represented a struggle not merely for territory but for the protection of national integrity and the rights of nations to exist free from invasion.

This theme is key to understanding the motivations behind the Allied response, as they aimed to restore and maintain peace by repelling the aggressors and stopping the chain of invasions that characterized the Axis Powers' approach to international relations during that period.