When does a country, or land, belong to somebody or a group of people?

Is it after they have moved onto the land and not been ejected for a period of time, perhaps a generation? Is it when some changes are made by the newcomers to the environment which will make it more suitable for the newcomer: by settled agriculture or nomadic pastoral management, or by some other device imposed upon the environment? Or is it by displacement of the previous owners or settlers, by either warfare or by mass migration? Or is it when the land is purchased from the...