Re: Islamically can you join a national army...
But the main Khalifa was same for all the countries. There were no borders with visa requirements.
Right, a Khalifa though has rarely ever had rule over the vast territory solely based on his native army. Far off lands are always ruled through treaties and political representatives. Even if we call it an empire such as the Ottomon empire, it never directly ruled over the territories that were included in the empire. The same culture in one corner of the empire is not common to all parts of the empire. The more far stretched a territory becomes the more hurdles come in treating as one piece of land. It automatically develops its own identity, if not named then, north, south, east and west. As long as your nationalism does not hinder the preservation of the higher cause to which you have allegience I see it as nothing wrong. If the muslim countries of today do not have armies with their nationalistic interest they would be ruled by some one else as the muslims in India.