Imperialism is the building of an empire. Expansion occurs when one state is more powerful than the other. Imperialism happens when one country or power, usually a bigger power, wants the resources in the smaller country.
Everything is similar all the way down to the leaders. "Colonel Miles Quaritch and King Leopold of Belgium are similar in their complete disregard for indigenous populations in pursuit of economic gain." in both cases their want for power and resources leads to their down fall.
A part human part Na'vi Avatar is created to get to know and understand the indigenous Na'vis from the planet Pandora and help them move away from there home for mining.
Imperialism was the want for power and the want to conquer all land that one could. it was an unending succession of empires. Every European country wanted control over different countries.