Source of slavery

From what I understand of the sources of slavery, a non-Muslim darbi living in a non-Muslim land can be legally enslaved by Muslims, even if there is no actual fighting happening. Does this mean that it would have been Islamically permissible to go into a non-Muslim land and simply round up the non-Muslim people living there and sell them as slaves like the Americans did to the Africans? This doesn't seem right but I also can't find evidence against it?