The Combined Forces
وَلَقَدْ كَانُوا۟ عَـٰهَدُوا۟ ٱللَّهَ مِن قَبْلُ لَا يُوَلُّونَ ٱلْأَدْبَـٰرَ ۚ وَكَانَ عَهْدُ ٱللَّهِ مَسْـُٔولًا
And they had already promised Allah before not to turn their backs and flee. And ever is the promise to Allah [that about which one will be] questioned.
Nearby ayahs help place verse 33:15 in sequence and meaning.
وَإِذْ قَالَت طَّآئِفَةٌ مِّنْهُمْ يَـٰٓأَهْلَ يَثْرِبَ لَا مُقَامَ لَكُمْ فَٱرْجِعُوا۟ ۚ وَيَسْتَـْٔذِنُ فَرِيقٌ مِّنْهُمُ ٱلنَّبِىَّ يَقُولُونَ إِنَّ بُيُوتَنَا عَوْرَةٌ وَمَا هِىَ بِعَوْرَةٍ ۖ إِن يُرِيدُونَ إِلَّا فِرَارًا
And when a faction of them said, "O people of Yathrib, there is no stability for you [here], so return [home]." And a party of them asked permission of the Prophet, saying, "Indeed, our houses are unprotected," while they were not exposed. They did not intend except to flee.
وَلَوْ دُخِلَتْ عَلَيْهِم مِّنْ أَقْطَارِهَا ثُمَّ سُئِلُوا۟ ٱلْفِتْنَةَ لَـَٔاتَوْهَا وَمَا تَلَبَّثُوا۟ بِهَآ إِلَّا يَسِيرًا
And if they had been entered upon from all its [surrounding] regions and fitnah had been demanded of them, they would have done it and not hesitated over it except briefly.
قُل لَّن يَنفَعَكُمُ ٱلْفِرَارُ إِن فَرَرْتُم مِّنَ ٱلْمَوْتِ أَوِ ٱلْقَتْلِ وَإِذًا لَّا تُمَتَّعُونَ إِلَّا قَلِيلًا
Say, [O Muhammad], "Never will fleeing benefit you if you should flee from death or killing; and then [if you did], you would not be given enjoyment [of life] except for a little."
قُلْ مَن ذَا ٱلَّذِى يَعْصِمُكُم مِّنَ ٱللَّهِ إِنْ أَرَادَ بِكُمْ سُوٓءًا أَوْ أَرَادَ بِكُمْ رَحْمَةً ۚ وَلَا يَجِدُونَ لَهُم مِّن دُونِ ٱللَّهِ وَلِيًّا وَلَا نَصِيرًا
Say, "Who is it that can protect you from Allah if He intends for you an ill or intends for you a mercy?" And they will not find for themselves besides Allah any protector or any helper.