Concerning the Multiplicity of Muslim Rulers
Refuting the doubt flung from many who ascribe themselves to al-Ikhwān or Ḥizb al-Taḥrīr that the āyāt and aḥādīth referring to the rulers are referring to the Caliph only.
So there is no harm in the multiplicity of Imāms and Sulṭāns, and it is obligatory for those people in whose land his orders and prohibitions become effective to give obedience to him after having giving bayʿah to him. It is likewise for the people of all the other regions.
All Praise is due to Allāh and Prayer and Peace upon His Messenger Muḥammad.
The Messenger (ṣallallāhu ʿalayhi wa-sallam) said: "The Khilāfah in my Ummah after me will be for thirty years. Then there will be kingship after that." 
Ibn Kathīr also reports a variation of this ḥadīth in his tafsīr: "The Khilāfah will last for thirty years. Then there will be powerful kingship."
The Messenger of Allāh (ṣallallāhu ʿalayhi wa-sallam): "Prophethood will be amongst you for as long as Allāh wills, then Allāh will raise it up when He wills, then there will be Khilāfah upon the way of Prophethood, then Allāh will raise it up when He wills, then there will be biting Kingship, then oppressive Kingship, then Khilāfah upon the way of the Prophethood."
The Messenger of Allāh (ṣallallāhu ʿalayhi wa-sallam) said: "The bonds of Islām will collapse, one by one. Every time a bond collapses the people will hold rigorously to the one follows it. The first one to collapse is the rulership and the last one to collapse is the prayer." 
From the above narrations it should become clear that the Khilāfah upon the Prophetic Methodology lasted only for 30 years. And hence whatever came thereafter (with perhaps the exception of the Khilāfah of ʿUmar ibn ʿAbd al-ʿAzīz) was kingship or tyrannical kingship. So from the Khilāfah of Yazīd up until this day of ours we have always had either a kingship or a tyrannical kingship. This is the reality, firstly.
Secondly, Al-Ḥāfiẓ Ibn Ḥajar said in ‘al-Fath’: "The Fuqahaa (Jurists) are in agreement concerning the obligation to give obedience to the Sultaan who took over them by force and also to perform Jihād along with him. [And that] obeying him is better than rebelling against him due to what this would contain of the shedding of blood and subduing of the common masses." 
Shaykh ʿAbd al-Lateef Ibn ʿAbd al-Raḥmān Ibn Ḥasan aalush-Shaykh said, "And the People of Knowledge… are unanimously agreed concerning giving obedience to the one who took over them by force in whatever constitutes goodness. They see it necessary to fulfill his orders and hold the correctness of his Imāmah, leadership. No two people [amongst them] differ concerning that. They also deem it impermissible to rebel against him with the sword and to divide the Ummah, even if the leaders are sinful, so long as they do not see clear open disbelief. The various texts [of these Jurists] from the four Imāms and other than them and their likes in this regard are [certainly] present." 
Shaykh al-Islām Muḥammad Ibn ʿAbd al-Wahhāb said: "The Imāms from every Madhhab are agreed concerning the one the forcefully took over a region or regions that he has the ruling of "Imām" in all matters. If this had not been so then the affairs of the world would never have been established. This is because for a very long time, before the era of Imām Aḥmad till this day of ours, the people have never gathered behind a single Imām. And they do not know anyone from the Scholars who has mentioned that any of the Sharīʿah rulings cannot be correct [effected, implemented] except by the overall Imām [the Khaleefah]." 
As-San’aanee, in explanation of the ḥadīth, "Whoever left obedience [to the Imām] and separated from the Jamāʿah and then died, then his is a death of Jāhiliyyah]", said: "His saying, ‘…left obedience…’, meaning obedience to the Khaleefah concerning whom there is agreement. And it is as if the intent here is the Khaleefah of a particular region because the people have never gathered together behind a single Khaleefah in all the lands of Islām since the time of the 'ʿAbbāsee State. Rather, the people of every region were independent with someone presiding over their affairs. If the ḥadīth was taken to mean the overall Khaleefah which the people of Islām had united behind, there would have been no benefit in it." 
Ash-Shawkaanee said, "As for when Islām spread and its territories expanded and its regions became distant [from each other], then it is known that in all of these regions loyalty was given to an Imām or Sultaan… So there is no harm in the multiplicity of Imāms and Sultaans and it is obligatory for those people in whose land his orders and prohibitions become effective to give obedience to him after having giving bay’ah to him. It is likewise for the people of all the other regions." 
The Shaykh and Imām, ʿAbd al-Lateef Ibn ’Abdur-Raḥmān Ibn Ḥasan aalush-Shaykh - may Allāh have mercy upon them all - said, in powerful words that uncover the confusing doubts in this topic and that refute the one who spreads them from amongst the ignoramuses: "… And those people - those who are under trial - do not know that with the exception of ʿUmar Ibn ʿAbd al-ʿAzīz and whoever Allāh willed from among the Banī Umayyah - great mishaps, insolence, taking up arms [against the people] and corruption occurred from most of those in charge [wullāt] of the people of Islām from the time of Yazīd Ibn Muʿāwiyah [till the present]. But along with that, the manner and behaviour of the notable scholars and mighty leaders with the rulers is well-known and renowned - they do not raise a hand against giving obedience in that which Allāh and His Messenger have commanded from among the legislated actions and obligatory duties of Islām.
And I will give you an example - that of al-Ḥajjaaj Ibn Yūsuf ath-Thaqafī, and his affair is well known in the ummah - that of oppression, repression, excessiveness in spilling the blood [of the Muslims], desecration of the Sanctities of Allāh, the killing of whomever he killed amongst the notables of the ummah such as Saʿīd ibn Jubair, the besieging of Ibn az-Zubair even though he had sought refuge in the Haram, and making lawful the sacred and sanctified, the killing of Ibn az-Zubair - even though Ibn az-Zubair had given obedience to him and the people of Makkah, Madīnah, Yemen, and most of 'ʿIrāq had given the pledge of allegiance to him [Ibn az-Zubair] and al-Ḥajjaaj was only a deputy of Marwaan, and then of his son ʿAbd al-Mālik and none of the khulafaa’ (successors) had given Marwaan a pledge and none of the influential people, those with power had given the pledge of allegiance to him. And along with all of this none of the People of Knowledge hesitated in obeying him and complying with him in that in which obedience is permissible from amongst the pillars of Islām and its obligations.
And Ibn ʿUmar and whoever met al-Ḥajjaaj were from amongst the Companions of Allāh’s Messenger (ṣallallāhu ʿalayhi wa-sallam), and they never contested with him and nor did they prevent obedience to him in that by which Islām is established and by which īmān (faith) is perfected. And it is likewise for those who were also in the era of al-Ḥajjaaj from among the taab’ieen such as Ibn al-Mūsáyyib, al-Ḥasan al-Baṣrī, Ibn Sīrīn, Ibrāhīm at-Taymee and those like them from among the leaders of the ummah.
And the affair continued like this between the leading scholars of the ummah - they would enjoin obedience to Allāh and His Messenger and making jihād in His path along with every leader [imām] whether righteous or sinful, as is well known in the books of the fundamental principles and beliefs of the religion.
And similarly, Banoo al-ʿAbbās, they conquered the lands of the Muslims forcefully, with the sword - and not one of the People of Knowledge and Religion aided them in that - and they killed hordes of people and many of the creation from among the Banoo 'Umayyah, their leaders and their deputies. And they killed Ibn Ḥubayrah, the ameer of 'ʿIrāq and they also killed Marwaan, the khalīfah - and it was reported that the murderers killed around eighty people from the Banoo 'Umayyah in a single day - and then they placed their blankets above the corpses, sat upon them and then called for food and drink.
So along with all of that the conduct of the leading scholars - such as al-‘Awzaa’ee, Mālik, al-Layth ibn Sa’d, ‘Ataa Ibn Abī Rabaah - with those kings is not hidden from the one who has a share in knowledge and realization. And then next generation of the People of Knowledge such as Aḥmad Ibn Ḥanbal, Muḥammad Ibn Ismāʿīl, Muḥammad Ibn Idrees, Aḥmad Ibn Nūḥ, Ishāq Ibn Rahawayh and their brothers … their occurred in their time what occurred from the kings of the great innovations and the denial of the Sifaat (Attributes of Allāh) and they were called to [affirm] these things and were put to trial by them] and whoever was killed, was killed such as Aḥmad Ibn Naṣr. But along with all of this it is not known that a single one of them raised his hand against obedience [to those kings] and that he saw fit to attack them…" 
Therefore, in light of the above it is necessary to give obedience to whomever is appointed over the Muslims, whether this occurs by their agreement or otherwise, in all the various lands. However this obedience is conditional in that it should be upon the ma’roof, (good) since there is no obedience to the creation in disobedience to the creation. This is the Salaf of the earliest times and those of current times are upon, including Shaykh Ibn ’Uthaymīn, Ibn Bāz (raḥimahullāh), Shaykk Ṣāliḥ al-Fawzān and Shaykh al-Albānī. In a cassette lecture Shaykh Ibn Bāz was asked concerning the rulers of some of the Muslim lands to day, such as the Middle Eastern countries, are they ‘Wullātul-Umoor’ and he replied in the affirmative, this cassette is in my possession.
Concerning the issue of ruling by other than what Allāh has revealed, then this is a great topic which cannot be discussed on the likes of this forum. I recommend that you await the release of "Fitnatut-Takfīr" of Shaykh al-Albānī. This contains the fatwá́s of Shaykh al-Albānī, Shaykh Ibn Bāz and Shaykh Ibn ʿUthaymīn on this great topic. And all of them are in agreement. Takfīr is not to be performed upon a ruler until it is known that he rejected Allāh’s rule as a matter of belief, or that he deemed secular law more superior to that of Islām, or he viewed Islām to be outmoded, backward etc. As for the one who replaces the Sharīʿah with secular law (tabdeel) then he too is not to be judged kāfir automatically until it is known that his action occurred from him due to his belief that secular law is superior etc. Likewise for the one who refers to secular law for judgement and makes it a reference point, he too is not to be judged a kāfir unless it is known from him that he holds this to be permissible as a matter of belief.
This is what the Shaykhs are upon and this is clear and manifest from there works. We finish below with just a few quotations but there are many more and I suggest you wait for the aforementioned book or refer to it in al-ʿArabīc:
Ibn al-Qayyim in Kitābur-Rūḥ - explaining the types of rule - so he mentioned the Shar’ul-Munazzal (the hukm of the Sharīʿah) and the Shar’ul-Mu’awwal (and this is the hukm of the scholars and mujtahids - and then he said:
"And as for the replaced law (al-hukmul-mubaddal) - and THAT IS RULING BY OTHER THAN WHAT ALLĀH HAS REVEALED - then it is not permissible to implement it nor to act by it, and it is not permissible to follow it, and the one guilty of it (saahibuhu) is BETWEEN (THE STATES) OF KUFR (DISBELIEF), FUSOOQ (REBELLION) AND DHULM (OPPRESSION)."
Shaykh Ibn ’Uthaymīn said, "And there is a doubt (Subḥah) with many of the youth, which has become firmly and deeply rooted in their minds and it has kindled the issue of revolting against the rulers - and it is: that those rulers replace the Sharīʿah (haa’ulā`il-Hukkaam MUBADDILOON), they prescribe the (secular) laws from themselves and they do not rule by what Allāh has revealed and the rule is present - but they prescribed laws from themselves. So (these youth) judged them with apostasy and disbelief and then they built upon this that so long as those rulers are disbeliever it is necessary to kill them, and does not matter that we are in a state of weakness. Because (not fighting) in the state of weakness was abrogated by the verses of the sword (permitting fighting), therefore there is no place for acting (as if we were) in the state of weakness - as they say - that state that the Muslims in Makkah were in!
So the answer to this doubt is that we say: "There is no escaping from that we know firstly: Does the description of apostasy apply to them or not? And this requires knowing the evidences which indicate that this saying or action is apostasy, then applying them to an individual, and then, whether this individual has any doubts (which may excuse him) or not? Meaning: Sometimes a text can indicate that this action is kufr and this saying is kufr, but there are preventive barriers which prevent the application of the ruling of kufr upon this specific individual… and these preventive barriers are many, amongst them dhann (speculation) which is ignorance and amongst them ghalabah (which means being overcome by something)…"
Then the shaykh gives some examples from the sunnah - so he mentions the ḥadīth of the man who asked for his body to be burned after his death and the ashes to be scattered across the earth. So the shaykh explains that the outward manifestation of his 'ʿaqīdah is kufr, and doubt in the power of Allāh. And the shaykh gives the example of the one who when he finds his lost camel in the desert says: "O Allāh, you are my servant and I am your Lord". So the shaykh says that this is a word of kufr, but the one who says it is not to be declared a disbeliever, because he was overcome (i.e. had no power of it) due to the intensity of his happiness. And the Shaykh also gives the example of "the one who is compelled to kufr - so he utters a word of kufr, or does an action of kufr, but he is not to be declared a disbeliever by a text of the Qurʾān, because he does not desire it and has not made it his choice." 
Shaykh Muḥammad Ibn Ibrāhīm aalush-Shaykh said: "The actualization of the meaning of 'Muḥammad is the Messenger (ṣallallāhu ʿalayhi wa-sallam) is from judging to his Sharīʿah and confining oneself to that and rejecting whatever is in opposition to that from amongst the rules (qawaaneen) and regulations and all those things for which Allāh has not revealed any authority. And these rules - the one who judges by them (hakama bihaa) or refers to them (haakama ilayhaa) - for judgement whilst believing in the correctness (sihhah) of that or the permissibility (to judge by them) (jawaaz), then he is a kāfir with the kufr that ejects from the Religion. And if he does that without belief (i'tiqaad) in their correctness and (regarding it) permissible to judge by them (jawaaz), then he is a kāfir with the kufr in action, which does not eject from the religion." 
Likewise he (raḥimahullāh) said: "The laws (al-qawaaneen) are kufr [i.e.] the belief (i'tiqaad) that they are permissible and are decisive (haakimah)." 
And also: "Whoever believes (i'taqada) in the permissibility (jawaaz) of ruling by what opposes the Rule of Allāh and His Messenger", and "believes (i’taqada) that other than the rule of the Messenger is better"
Also from Shaykh al-Islām Ibn Taymīyyah "or (the one) who replaced the Sharīʿah (baddala ash-Shar’) - that [from it] which is agreed upon - he is a kāfir, an apostate by agreement of the jurists..." 
Further on he says: "And the word Shar' it is used - in the understanding of men - with three meanings..." so he mentions Shar’ul-Munazzal (that which is revealed -the Book and the Sunnah) and Shar’ul-Mu’awwal (the opinions of the scholars etc...) and then said:
"And the third: Shar’ul-Mubaddal - and this is lying against Allāh and against His Messenger or upon the people with a false testimony and its likes, and clear oppression. So whoever says: 'Indeed, this is from the Shar’ of Allāh (i.e. a particular ruling a law), then he has disbelieved - there being no doubt or dispute in this - such as the one who says: 'That consuming blood and the dead animal is lawful'". 
So the definition of tabdeel that ejects from the religion is introducing/changing laws and then claiming/believing that what has been introduced is from Allāh! Both things have to be their, the action and the belief for takfīr to apply.
In fact in the same statement [3/267] the completion of his words shows the real intent of his words:
"or (the one) who replaced the Sharīʿah (baddala ash-Shar') - that [from it] which is agreed upon - he is a kāfir, an apostate by agreement of the jurists and it is regarding the likes of this that the verse was revealed - according to one of two sayings: "and whoever does not judge by what Allāh has revealed, they are the disbelievers" - meaning that it is the one who holds the belief (or declares) ruling by other than what Allāh has revealed to be permissible..."
There is much more but I hope this will suffice to remove the various doubts and misconceptions which are often propagated by the political activists of today.
 Ṣaḥīḥ – Reported by Aḥmad, Tirmidthī, Abū Ya’lā.
 ]Reported by Imām Aḥmad and Ibn Hibbān. Ibn Hibbān put it under the chapter heading, "A mention of the narrations that the first appearance of the breakdown of the bonds of Islām will come from the corruption of the rule and the rulers."
 Related in Fat‘h al Bārī (7/13).
 Majmūʿur Rasāʾil wal-Masaa’ilun-Najdiyyah (3/128)
 ad-al-Durar al-Sunniyyah fil-Ajwibatun-Najdiyyah (7/239)
 Subulus-Salām Sharḥ Bulūgh al-Marām min Adillatil-Aḥkām (3/499)
 as-Saylul-Jarraar (4/512)
 ad-al-Durar al-Sunniyyah fil Ajwibatun-Najdiyyah (7/177-178).
 al-Taḥdhīr minFitnah al-Takfīr of Shaykh Nāṣir (p. 105-107).
 Fatāwá Shaykh Ibrāhīm (10/9)
 Fatāwá Shaykh Ibrāhīm (12/280)
 Fatāwá Shaykh Ibrāhīm (12/280-291)
 Fatāwá Shaykh Ibrāhīm (3/267)
 Fatāwá Shaykh Ibrāhīm (3/268)