Simply understood, this means a thing that is obvious, and whose occurrence in the future cannot be prevented.

It was a phrase made up and then seized upon by European settlers to justify their rapid expansion from the eastern coast of what is now the United States, to eventually the west and far south. This was a process that was to involve an explicit and conscious genocidal programme against all the peoples living in those areas.

The phrase helped the expanding Europeans to convince themselves that there was nothing that could been done to prevent all these occurrences.
So it was justifiable because it was inevitable, and inevitable because it was justified. Perfect circular reasoning that feeds racist thinking and plagues the earth up to today.