The correct answer to this open question is the following.
The topic that is often used as a theme for studying world history since World War 1 is "Decolonization and Nationalism."
When World War 1 hostilities ended with the defeat of Germany and its allies, people from European and African nations demanded the possibility of self-governance. Freedom and democracy were two words that people related to in order to have a better form of government. Many European superpowers such as Britain and France still had colonies in Africa, when more and more people raised their voice to demand decolonization and the formation of new governments based on nationalistic ideals.