Surah Al-Anfal — Ayah 73 (8:73)
وَٱلَّذِينَ كَفَرُواْ بَعۡضُهُمۡ أَوۡلِيَآءُ بَعۡضٍۚ إِلَّا تَفۡعَلُوهُ تَكُن فِتۡنَةٞ فِي ٱلۡأَرۡضِ وَفَسَادٞ كَبِيرٞ ٧٣
English translation (Saheeh International)
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.
Brief commentary
The disbelievers are united in their disbelief and they therefore become friends of one another. A believer should not become friends with them. If you do not become friends with the believers and oppose the disbelievers, the believers will suffer weakness as a result of a lack of support and there will be great corruption on earth as people will be prevented from Allah’s path.