http://sultanknish.blogspot.com/
Once upon a time when you wanted to invade another people’s land, you built your longboats, got together some young males with few prospects at home and set sail for greener pastures. Today the longboats are jet planes and the invaded use them to ferry over their own invaders.
Colonization usually required three elements. Surplus male population, the technology to make the journey and the means to subjugate or drive off the people already living there.
Western medicine and aid have helped create and sustain a surplus population which has nowhere to go and nothing to do in its own highly stratified societies. At home they start Arab Spring like revolutions, which is one more reason for China’s One Child policy. With few economic opportunities in an oligarchy, they have to move elsewhere to get ahead.
The ability to colonize distant parts of the world usually required a certain level of sophistication. Any barbarian could walk a few miles, grip an axe and smash the heads of the equally backward people who lived there. But eventually they would either come up against a more sophisticated empire or the sea, natural barriers that they couldn’t cross without advancing further up the cultural ladder.