The following is from the draft english translation of the Arabic book مقدمة الدستورأو الأسباب الموجبة له (Introduction to the constitution and the evidences that make it obligatory) published by Hizb ut-Tahrir 1382 Hijri (1963 CE). Please refer to the original Arabic for accurate meanings. Please note some of the adopted opinions of the Hizb have changed since the time the book was published so any of the adopted literature published after this book which contradicts what is mentioned in this book abrogates those specific points
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No one can be Khalifa unless the Muslims appoint him, and no one possesses the mandatory powers of the leadership of the State unless the contract with him has been concluded according tothe Shari’ah, like any contract in Islam.
The evidence is that the Khilafah is a contract upon satisfaction and consent, since its reality as a contract means it is not contracted except through two contracting parties, and therefore no one is the Khalifah unless he was appointed to it by those whose agreement completes the conclusion of the contract according to the Shari’ah. So if someone appoints himself Khalifah without the pledge from those whom the Khilafah is contracted through, then he would not be a Khalifah until his pledge occurs with choice and consent from those whom the conclusion of the contract takes place. So the fact that the Khilafah is a contract necessitates the presence of two contracting parties, with each of them having the necessary Shari’ah qualifications to be entrusted with the contract and conclude it.
If a conqueror came about and took the ruling by force he does not become a Khalifah by that, even if he announces himself as Khalifah of the Muslims, since the Khilafah was not contracted to him by the Muslims. If he took the pledge of allegiance from the people by force and compulsion, he does not become the Khalifah even if he was given the pledge, since the pledge given through compulsion and force is not considered and so the Khilafah cannot be contracted by it. This is because a contract of choice and consent cannot be completed through compulsion and force, and so it is not contracted except through a pledge given with satisfaction and consent. However, if this conqueror managed to convince the people that it was in the benefit of the Muslims to give him the pledge, and that the implementation of the Shari’ah would be complete through giving the pledge to him – and so the people became convinced and satisfied with that and gave him the pledge of allegiance on that basis with their own choice, then he would become the Khalifah from the moment that he was given that pledge by the people freely even though he took the authority through force and power. Therefore, the condition is the contracting of the pledge, and this is only reached through consent and choice, irrespective of whether the one who reached it was the ruler and leader, or wasn’t.
It is stipulated that the authority of the region or the country that gives the Khalifah a contracting pledge is autonomous dependent upon the Muslims alone, and not upon any disbelieving state; besides the security of the Muslims in that country, both internally and externally, is by the security of Islam not the security of the disbelief. With respect to the pledge of obedience taken from other countries, there are not such conditions.
The evidence is the forbiddance of the disbelievers having authority over the Muslims, in accordance with the His (swt) words “Allah will never grant to the disbelievers a way over the believers” (TMQ 4:141), so if the authority of the disbelievers over the Muslims is present in any part of the Islamic lands, then that land would not be suitable to establish the Khalifah, since the establishment of a Khalifah is simply the establishment of an authority. Since that land does not possess the authority it therefore cannot give it. Also its authority is an authority of disbelief, and the Khalifah is not established with the authority of disbelief.
This is from the angle of the authority; as for the issue of security, its evidence is the evidence for Dar Al-Islam and Dar Al-Kufr, since the establishment of the Khalifah would make the abode into an abode of Islam, and it is not possible for an abode to be an abode of Islam simply by establishing the rule of Islam but rather it is imperative that its security is by the security of Islam and not that of disbelief, since the conditions for the abode to be considered an abode of Islam are: firstly, to be ruled by Islam and secondly, for its security to be the security of Islam and not the security of disbelief.