The Disbelievers are Allies of Each Other; the Muslims are not their Allies

After Allah mentioned that the believers are the supporters of one another, He severed all ties of support between them and the disbelievers. In his Mustadrak, Al-Hakim recorded that Usamah said that the Prophet said,

«لَا يَتَوَارَثُ أَهْلُ مِلَّتَيْنِ، وَلَا يَرِثُ مُسْلِمٌ كَافِرًا، وَلَا كَافِرٌ مُسْلِمًا»

(No followers of two religions inherit from each other. Therefore, neither a Muslim inherits from a disbeliever nor a disbeliever from a Muslim.)

The Prophet recited this Ayah,

وَالَّذينَ كَفَرُواْ بَعْضُهُمْ أَوْلِيَآءُ بَعْضٍ إِلاَّ تَفْعَلُوهُ تَكُنْ فِتْنَةٌ فِى الاٌّرْضِ وَفَسَادٌ كَبِيرٌ

(And those who disbelieve are supporters of one another, (and) if you (Muslims) do not do so (protect one another), there will be Fitnah on the earth, and great corruption.) Al-Hakim said, "Its chain is Sahih, and they did not record it." However, the following, from Usamah bin Zayd, is in the Two Sahihs; the Messenger of Allah ﷺ said,

«لَا يَرِثُ الْمُسْلِمُ الْكَافِرَ وَلَا الْكَافِرُ الْمُسْلِم»

(Neither a Muslim inherits from a disbeliever nor a disbeliever inherits from a Muslim.)

Allah said next,

إِلاَّ تَفْعَلُوهُ تَكُنْ فِتْنَةٌ فِى الاٌّرْضِ وَفَسَادٌ كَبِيرٌ

(If you do not do so, there will be Fitnah and oppression on the earth, and a great corruption), meaning, if you do not shun the idolators and offer your loyalty to the believers, Fitnah will overcome the people. Then confusion polytheism and corruption will be rampant, for the believers will be mixed with disbelievers, resulting in tremendous, widespread trials corruption and mischief between people.