Some people are nomads and hence they migrate naturally. For example, the travellers or Romani originated from India and they travelled through Egypt - hence their colloquial name, (e)gypsies to Europe. That'a some epic migration. Quite a number of them keep to this lifestyle, though I suspect that the numbers adhering to this lifestyle in Europe have dwindled.
This is quite different from the European invasion of the Americas aftwr Columbus stumbled upon that continent. There, an essentially sedentary people invaded another continent and settled it. Hence Settler-Colonialism. They kept going as they had guns and germs and spread throughout that whole continent. (See Jared Diamonds Guns, Germs and Steel by Jared Diamond).
