Indeed of those who belonged to Islam and first to ever to fall into the kuffr of ruling other then the Shariah where the tartars in the end of the 7th century Hijri. They where those who accepted Islam after the invasion and ransacking of the Muslim Lands when they where pagans.

As far as the fataawa of Ibn Taymiyya is concerened, it is his saying may Allah have mercy upon him, in his Fataawa, regarding the Tartars, Majmua’ Al Fataawa, Vol 28 P524 :

“And it is known from the Deen of Islam by necessity and by the consensus of all Muslims that whoever legalizes to follow other that the Shari’ah of Muhammed (saw) he is a Kafir. And his kuffr is similar to that of the one who believes in some part of the Quran and rejects parts of it.

As Allah (swt) said: Verily those who disbelieve in Allah and his Messengers and wish to make a distinction between Allah and his Messengers (by believing in Allah and disbelieving in his Messengers) by saying ‘we believe in some but reject others’, and wish top adopt a way in between, they are in truth disbelievers. And we have prepared for the disbelievers a humiliating torment. [EMQ An Nisa 4:150-151.]

As for the fatawa of Ibn Kathir wherein he related the Ijma’ on the kufr of the one who rules by other then what Allah has revealed, he stated in Al-Bidayah Wan-Nihayah Vol 13, P119:

So Whosoever abandons the wise Shariah which was revealed upon Muhammed ibn Abdullah, the seal of the prophets, and goes to other abrogated Shariah for judgment, he becomes a Kafir. So how about the one who goes to Al-Yasa (man-made law) for judgment and gives it precedence (over the Sharia of Muhammed (saw)). Whosoever does this has become a Kafir by the Ijma of the Muslims.

An He (swt) said: “Do they then seeek the judgment of (the days of) Ignorance? And who is better in Judgment than Allah for a people who have a firm faith.” [al-Maidah 5:50] He (swt) also said: “But no by your Lord they can have no faith, until the make you (O Muhammed (saw)) judge in all disputes between them and find no resistance against your decisions, and accept them with full Submission”. [an Nisaa 4:65]

Some of those who argue in favour of the Tawagheet rulers have claimed that it is irrelevant to apply the fatawaas of Ibn Taymiyyah and Ibn Kathir regarding the tartars to todays rulers, because the Tartars were pagans. But this in incorrect for indeed the tartars embraced Islam, but the kept ruling other than its Shariah, and these equalities are the very same equalities of today’s Tawagheet, therefore their verdict is like theirs, and their situation is like theirs. In fact the reality is that today’s rulers are far greater in kuffr and deviance then the Tartars, and the Muslims of today are in a more worse situation more severer in oppression and darkness then them.

The scholars Sheikh alHaafiz `Umar `Abdur-Rahmaan, Sheikh Naajah Ibrahim, Sheikh `Isaam ud-Din al Darbaalah and Sheikh `Aasim `AbdulMaajid had this to say on the topic of the rulers over the Muslim lands in this era,

“And the rulers of today have given their allegiance to the east and the west, both of which are kaafir. And all of the love is given to the Jews and the Christians. The hatred with pure animosity, warfare and plotting and deception are used on Islam and its people. And they have in this time left the judgment of the book of Allah. They have replaced the Divine Revealed Law and on top of all of that, they claim that they are Muslims. They also have the evil scholars working for them, giving them the title of ‘khalifa’ and alHaakim bi amr illah (‘The Law giving legislating Judge by the order of Allah’).

The brothers continue on in another place,

“Thus whoever legislates besides Allah and replaces the Shari`a of Allah in another legislation, then already he has opposed Allah in His legislation. He has made himself a partner with Allah and a rival and he has left from the fold of Islam. Going out from them is compulsory and they are the rulers of today, who have apostated.”