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
It is the Khalifah who deputises for the Ummah in authority and in implementing the Shari'ah.
Khilafah is the general leadership of all the Muslims in the world, charged with the implementation of Shari'ah and of carrying the Islamic Da'awah to the whole world. It is the Muslims who appoint he who assumes this leadership, i.e. who appoint the Khalifah. Since the authority belongs to the Ummah and since the implementation of Shari'ah is an obligation upon the Muslims and the Khalifah is their leader, thus his reality is that he acts as a deputy for them in authority and in the implementation of Shari'ah.
Hence, he does not become a Khalifah unless the Ummah gives him the Baya'a. the Baya'a of the Ummah serves as evidence about the fact that he is a deputy for her; also, the fact that obeying the Khalifah is obligatory serves as evidence that this Baya'a, through which the contracting of the Khilafah takes place, stipulates the handing of authority to him. This means that he acts as the deputy of the Ummah in authority. It is on this basis that this article has been drafted.
The Khilafah is a contract based on consent and choice. Hence, none should be coerced into accepting it and none should be coerced into choosing he who assumes it.
Evidence about this article is the same evidence about any legitimate contract concluded between two contractors. This is so because it is a Shari'ah contract like any other.
Furthermore, the Hadith of the desert Arab who gave his Baya'a to the Messenger of Allah (saw) and then came back asking him to be discharged of his Baya'a, but the Messenger of Allah (saw) refused; this serves as evidence about the fact that the Khilafah is a contract:
Jabir Ibnu Abdullah (r) reported that a desert Arab gave his Baya'a to the Messenger of Allah (saw) over Islam; then he felt a malaise, so he said to the Messenger of Allah (saw): Discharge me of my Baya'a. The Messenger of Allah (saw) refused. So he repeated his request and said: Discharge me of my Baya'a. the Messenger of Allah (saw) again refused. Then the desert Arab left. Upon this the Messenger of Allah (saw) said: Madinah is like the bellows. It banishes its filth and its scent becomes pure.
Since the Baya'a of the Khilafah is a Baya'a of obedience to he who has the right of obedience in relation to assuming the rule, it should then be a Baya'a of consent and choice. It would be invalid if it were taken by way of coercion, be it the coercion he who gives the Baya'a or he who takes the Baya'a.
This is so because the Messenger of Allah (saw) said: "Blame has been lifted from my Ummah in case of error, forgetfulness and coercion."
This is general, it applies for every type of contract, including the Khilafah contract. Hence, every contract that has been drawn by way of coercion is null and void, because it would not have been contracted properly. Likewise is the Khilafah; it cannot be contracted by way of coercion, i.e. just like all the other types of contracts. Also, the Khilafah could be concluded unless there were two contracting parties, just like any other type of contract. Hence, no one could become Khalifah unless someone appoints him to assume the Khilafah. Therefore, if he were to appoint himself as Khalifah without a Baya'a taken from those with whom the Khilaah is contracted through their Baya'a, he could not become a Khalifah, unless they give him their Baya'a with full consent and free choice; in this case he becomes Khalifah after the Baya'a, and not before it. If he were to coerce them into giving him the Baya'a, he would not become a Khalifah through this Baya'a which he had taken by way of coercion and the Khilafah would not be contracted with him through this Baya'a; because it is a contract and it cannot be concluded by coercion because the Messenger of Allah (saw) said: Blame has been lifted from my Ummah in case of error, forgetfulness and coercion.
As for the dominant Sultan who usurps the rule by force, he would not become Khalifah simply by seizing power by force. He however becomes a ruler and in case, the matter would require careful consideration. If rebellion against him and returning the authority to the Ummah would not lead to a strife between the Muslims, then he should be fought and the authority should be returned to the Ummah; because he would be considered as a usurper of authority from the Ummah and this action of his would be a Munkar that has to be removed, and its removal would be a duty of sufficiency upon the Ummah. On the other hand, if rebellion against him to return the authority to the Ummah would lead to a bloody strife in the country, in this case, it would be forbidden to rebel against him because to cause a strife between the Muslims is Haram, thus what leads to strife would be Haram according to the principle that states:
The means that leads to Haram is Haram. In this case, his obedience becomes obligatory and Jihad under him becomes obligatory as well to quell the strife, unless he did not establish prayer and flagrant Kufr became manifest, i.e. unless he did not rule by Islam, in which case he must be fought in order to restore the rule of Islam.
It was mentioned in Al-Fatah: "The scholars have agreed upon the obligation of obeying the dominant Sultan and of performing Jihad with him, also that his obedience is better than rebellion against him because this would avert a bloodshed and would pacify the masses. They only exempted from this the flagrant Kufr perpetrated by the Sultan, in which case it would be forbidden to obey him; in fact challenging him and even fighting him becomes obligatory upon those who are capable of doing so as mentioned in the Hadith." In any case he remains an Amir but he would not become a Khalifah unless he takes the Baya'a from people with full consent and choice, and without any coercion.
This is the meaning of the wording in this article: The Khilafah is a contract based on consent and choice.