It is something that the jurists determined during the time when the fiqh was being recorded. Dividing the world into two or three categories: Dar-al-Islam, Dar-al-Harb and Dar-al-Ahd (a country in which there is a treaty between it and the Muslim state) is not a Shari’ah ruling. It’s something the mujtahideen developed for the reality they were facing, and thus its rulings are not absolute and unchangeable.
Indeed, the ijtihaad was influenced by the political situation in which jurists lived, at a time when the Muslim nation was united, powerful, spreading and ruled on the basis of the supremacy of the Shari’ah.
The political situation is tremendously different today. The categorization is primarily connected to the Ahkaam of Jihad, and the available fiqh developed under conditions when clear differentiation was more easily discernible.
Obviously, when the Muslim lands are no longer ruled according to the supremacy of the Shari’ah, with laws derived both from Islamic and from non-Islamic sources, and when the Muslim lands do not, in fact enjoy economic sovereignty or political independence; this not only allows, but requires a new ijtihaad that responds to the new reality.