How did aggression cause World War 2?

The World War 2 definition of aggression was the same as it is today.

An action of a nation or state violating by force or attack the rights of another state or nation, especially their territorial and human rights. An unprovoked offensive, invasion or other action is considered an aggressive act which the military is prepared to stop or rather repel.

An aggressor is a force who initiates combat action, especially the invasion of another nation, against a nation, people group or property.