Hadith Prohibiting Taking The Lands Of Others And Invasions Of Sahaba
The companions of the prophet were instructed by him to conquer the neighboring countries to protect the center of Tawheed, i.e. Hijaz. In the time of Hazrat Umar, all those intended lands were brought under the flag of the caliphate. After that, the conquests of other lands were a political necessity. That era was the era of power. Whoever had more power than his neighbors, would definitely conquer them. Therefore Muslims too have to do the same to protect their lands, and yes, some Muslim caliphs and kings conquered other lands just to exercise their power as was the fashion of that time which is ethically not justified but a political necessity.