I guess it's legal if you kill all the people
It's called conquering.
It has been done for centuries now.
Britain did it, germany did it, russia did it, japan did it, etc.
We are at a more civilized society now and conquering doesn't happen anymore
by force.
We now conquer countries by buying them out. We loving bought alaska. That stuff isn't even our actual territory.
Muslims conquer countries by over populating the area with the gay ass refugee boats they came on then when they become the majority they will have enough power to enact sharia law and start claiming islamic states.