Spanish Conquistadors

The first is more political and economical rather than cultural - money and power. Most of empires, states and civilizations have allways tried to invade others, or at least spread their influence to get economical and policital profit. With the western culture the best historical examples are probably conquest of The New World, Africa, Australia, and so on. They didn't go there mainly to dominate the native cultures, but to gain money from local resourses and political power.

Now we might ask why didn't they "just" exploit the land and leave the native cultures be. Which brings me to the second reason, which is more from a cultural perspective. Example: Why did Spanish conquistadors think the natives in Sounth America couldn't be human and