In Islam the concept of “country” has been replaced with the concept of “land” and instead of “government”, the “ummah”. The land of Islam is one and contractual, conventional borders and linings have no impact on its sole essence. In an ideal situation this single land is supposed to be ruled by an infallible Imam (A.S.) and the universal government of Imam Mahdi (A.F.) is the manifestation of this belief. If the grounds were present in the time of the occultation of the infallibles (A.S.) and it was in the favor of the Islamic land to be ruled as a whole, the faqih would be the one responsible for managing these lands. But, if it is better to govern every area of this land by a faqih – especially a native faqih - than this method may be carried out. Therefore as a global and fundamental principle in the political doctrine of Islam the Islamic land is one and single, but can be separated and broken up into smaller lands and different countries, states or cities of a country, or even be governed under any other kind of setup; this is something that is to be decided by political factors and mechanisms.
Today, with contemporary geographical borders, the world has been divided into different countries that have been approved and accepted by their governments and have evidently separated these countries from each other, giving everyone of them their specific governors. Those who reside in a country are, based on special conditions, called “citizens” and others are called “foreigners”. Some laws are confined to citizens while a group of laws are confined to foreigners and some other are meant for both citizens and foreigners. Are these geographical borders significant in Islam or not? In other words, what is Islam’s perspective regarding the world?
To answer to this question we must say: Islam introduces itself as a global religion which is for all eras and ages and believes that the whole world is its land in which it must rule and be propagated. Areas where the majority of residents are Muslim are named “Balad al-Islam” or “Dar al-Islam” (The Islamic Land) while the other parts of the world are called “Balad al-Kufr” or “Dar al-Kufr” (The Land of Kufr). Hence the sole border and boundary in Islam is man’s faith and beliefs; moreover the only accepted belief is Islam. "إن الدین عند الله الإسلام" Instead of the concept of “country”, Islam uses the term “land”, and uses “Ummah” in place of the word “government/state”. That is, racial and national differences are not of any value and importance as opposed to belief and faith.
The Islamic land is one and single, meaning that contractual and formal borders cannot alter its sole essence. This one land, in an idealistic scenario, is ruled by an infallible Imam (A.S.) and the global reign of Imam Mahdi (A.F.) that is to take place during his return is the manifestation of this belief. In the time of the occultation of the Imam (A.S.) if the grounds for establishing a government are at hand and governing all Islamic states as a union (i.e. one land) is possible, then a qualified faqih must accept the responsibility and do so. However, if this isn't the case and it is better to assign a faqih, as governor – especially a native faqih -, to every portion of the land than this method may be carried out. Therefore as a global and fundamental principle in the political doctrine of Islam the Islamic land is one and single, but can be separated and broken up into smaller lands and different countries, states or cities of a country, or even be governed under any other kind of setup that suits best.
Others books you can study:
1. Mahdi Hadavi Tehrani, Velayat va Diyanat, published by “Khaneye Kherad”, Qom, second edition, 1380.
 Ale Imran:19.