Why don't nations unite with one another?

Division is a torment, and unity is a mercy. The Quran and Sunnah have urged unity and warned against division. Allah Almighty says: "And do not be like those who divided and differed after clear proofs had come to them. And for them is a great torment." The principle is that Muslims should be in one state under a general leadership, and this is what Muslims were under the Islamic Caliphate. Due to the importance of the Caliphate, scholars have annexed the obligation of appointing a general Imam to the creeds. Muslims remained under one leadership until Western colonialism came and divided their lands into weak mini-states, and sowed enmity among them. There is no way for Islamic countries to unite as long as the disbelievers control them. Agreements like Sykes-Picot are what prevent Islamic countries from uniting. The most important factor in the disunity among Muslims is the control of external powers, in addition to other superficial factors. The only way for unity to return is to return to Allah Almighty and adhere to His comprehensive religion.