Al-Anfaal 8:73

Those who disbelieve are protectors of one another[46]. Unless you do likewise, there will be persecution in the land and great corruption.

[46] i.e., taking one another as allies and abandoning all bonds with the disbelievers.

And those who disbelieved are allies of one another. If you do not do so [i.e., ally yourselves with other believers], there will be fitnah [i.e., disbelief and oppression] on earth and great corruption.

73. And those who disbelieve are allies of one another, (and) if you (Muslims of the whole world collectively) do not do so [i.e. become allies, as one united block under one Khalifah (a chief Muslim ruler for the whole Muslim world) to make victorious Allâh’s religion of Islâmic Monotheism], there will be Fitnah (wars, battles, polytheism) and oppression on the earth, and a great mischief and corruption (appearance of polytheism).[8]

[8] (V.8:73) It has been mentioned in Tafsjr At-Tabari, that the best interpretation of this Verse: - ["And those who disbelieve are allies of one another, (and) if you (Muslims of the whole world collectively) do not do so (i.e. become allies, as one united block V.8:73)."] is "That if you do not do what We (Allâh) have ordered you to do, [i.e. all of you (Muslims of the whole world) do not become allies as one united block to make Allâh’s religion Islâm) victorious, there will be a great Fitnah (polytheism, wars, battles, killing, robbing, a great mischief, corruption and oppression.)"]. And it is Fitnah to have many Khalifahs (Muslim rulers), as it has been mentioned in Sahih Muslim by ‘Arfajah, who said: I heard Allâh’s Messenger صلى الله عليه وسلم saying: "When you all (Muslims) are united (as one block) under a single Khalifah (chief Muslim ruler), and a man comes up to disintegrate you and separate you into different groups, then kill that man."
Also there is another narration in Sahih Muslim: Narrated Abu Sa‘îd Al-Khudrî رضي الله عنه: Allâh’s Messenger صلى الله عليه وسلمsaid: "If the Muslim world gave the Bai‘a (pledge) to two Khalifah (chief Muslim rulers), the first one who was given the Bai‘a (pledge) first will remain as the Khalifah, then kill the latter (the second) one." So it is a legal obligation, from the above-mentioned evident proofs (from the Qur’ân and the Prophet’s statement), that there shall not be more than one Khalifah (a chief Muslim ruler) for the whole Muslim world or otherwise there will be a great Fitnah (mischief and evil) amongst the Muslims, the ultimate results of which will not be worthy of praise.

And those who disbelieve are protectors one of another - If ye do not so, there will be confusion in the land, and great corruption.