Does Allah favour a nation to fight against another i.e., favouring some people over others?

Well, yes! Allah favours some people over others, as Allah favours some individuals over others. There are numbers of verses in the Quran where Allah speaks about some people being favoured over others by Him. Even some Prophets were favoured more than other Prophets. Therefore, this concept is not strange, because Allah repels some people and favours others, as He uses them to repel many people and to save others.

(تِلْكَ الرُّسُلُ فَضَّلْنَا بَعْضَهُمْ عَلَىٰ بَعْضٍۘ مِنْهُمْ مَنْ كَلَّمَ اللَّهُۖ وَرَفَعَ بَعْضَهُمْ دَرَجَاتٍۚ وَآتَيْنَا عِيسَى ابْنَ مَرْيَمَ الْبَيِّنَاتِ وَأَيَّدْنَاهُ بِرُوحِ الْقُدُسِۗ وَلَوْشَاءَ اللَّهُ مَا اقْتَتَلَ الَّذِينَ مِنْ بَعْدِهِمْ مِنْ بَعْدِ مَا جَاءَتْهُمُ الْبَيِّنَاتُ وَلَٰكِنِ اخْتَلَفُوا فَمِنْهُمْ مَنْ آمَنَ وَمِنْهُمْ مَنْ كَفَرَۚ وَلَوْشَاءَاللَّهُ مَا اقْتَتَلُوا وَلَٰكِنَّ اللَّهَ يَفْعَلُ مَا يُرِيدُ) {البقرة:253}

“Those messengers – some of them We caused to exceed others. Among them were those to whom Allah spoke, and He raised some of them in degree (of honour). And We gave Jesus, the son of Mary, clear proofs (Signs), and We supported him with the Pure Spirit [i.e., Gabriel]. If Allah had so willed, succeeding generations would not have fought among each other, after clear proofs (signs) had come to them, but they differed, and some believed and others disbelieved. If Allah had so willed, they would not have fought each other; but Allah does what He intends.”[1]

Reference:

[1] Surah al-Baqara, verse 253.