The Quran

لا يُقاتِلونَكُم جَميعًا إِلّا في قُرًى مُحَصَّنَةٍ أَو مِن وَراءِ جُدُرٍ ۚ بَأسُهُم بَينَهُم شَديدٌ ۚ تَحسَبُهُم جَميعًا وَقُلوبُهُم شَتّىٰ ۚ ذٰلِكَ بِأَنَّهُم قَومٌ لا يَعقِلونَ

Muhammad Asad

Never will they fight you, [even] in unison, otherwise than from within fortified strongholds or from behind walls.¹ Severe is their warlike discord among themselves: thou wouldst think that they are united, whereas [in fact] their hearts are at odds [with one another]: this, because they are people who will not use their reason.²

The meaning is: "Even if they were able-which they are not - to put forth against you a truly unified front, they will always fight you only from what they regard as well-established 'positions of strength'."
Sc., "with a view to achieving what is good for themselves": implying that people who have no real faith and no definite moral convictions can never attain to true unity among themselves, but are always impelled to commit acts of aggression against one another.