How did Europeans justify the cost, morality, and effort of their imperialist policies?

There were a number of different motivations and justifications for European imperialism, but they can generally be boiled down to five basic categories:

1. Economic: Many European powers perceived that they could make themselves much wealthier by plundering from countries too weak to defend themselves.

2. Exploratory: Some European imperialism began as an honest attempt to explore and understand the world beyond what they knew.

3. Ethnocentric: Many European cultures believed that they were superior, "more civilized", and that other societies would benefit from adopting their way of life.

4. Political: Colonizing other countries gave European countries power and prestige that was valuable in their conflicts with other European countries.

5. Religious: Europe was predominantly Christian, while most of the world was not; some European Christians felt it was their holy duty to spread Christianity around the world.

Of these, the economic motive was probably the strongest, yet paradoxically it made the least sense. Imperialism is actually much less economically efficient than mutual trade, because mutual trade maximizes the productivity of both countries, creating more wealth for all. But people did not generally understand this, and instead thought that there was more or less a fixed amount of wealth in the world, so the best way to get more was to take from someone else. While this is largely true of some resources (such as land), it is definitely not true of many other forms of wealth (such as manufactured goods).