Short answer
Ahzab means "the Confederates" or "the allied groups". The Battle of Ahzab is the same as the Battle of the Trench (Khandaq), in Shawwal 5 AH (627 CE), when Quraysh and allied tribes besieged Madinah. Surah 33 is named al-Ahzab after it.
The Sunni account is that the Ahzab were the allied forces of Quraysh, Ghatafan and other tribes, joined by Banu Qurayza from inside, who besieged Madinah for about a month. Allah sent a wind and unseen armies, and they left without a pitched battle.
The four schools agree on this.
Explanation
Hizb means a party or group, and ahzab is its plural. In the seerah it names the coalition that marched on Madinah: Quraysh, Ghatafan and other Arab tribes, encouraged by leaders of the exiled Banu Nadir. The Muslims call the same event the Battle of the Trench, because they dug a trench across the open northern side of Madinah. The biographers say the idea came from Salman al-Farisi.
The siege lasted about a month in the cold of winter, with little food. Surah al-Ahzab describes it: armies came from above and below, eyes rolled with fear and hearts reached the throats. The hypocrites made excuses, while the true believers said: this is what Allah and His Messenger promised us.
Then Allah sent a cold, fierce wind that tore down the enemy's tents and put out their fires, and the coalition broke up and went home. The Prophet ﷺ used to praise Allah who "defeated the Confederates alone". After it he said that Quraysh would not attack them again; from then on the Muslims would go to them.
The full story, including the trench, the miracles during the digging and the events with Banu Qurayza, is on Battle of the Trench (Khandaq).
The evidence
Quran, Surah Al-Ahzaab 33:9
يَٰٓأَيُّهَا ٱلَّذِينَ ءَامَنُوا۟ ٱذْكُرُوا۟ نِعْمَةَ ٱللَّهِ عَلَيْكُمْ إِذْ جَآءَتْكُمْ جُنُودٌۭ فَأَرْسَلْنَا عَلَيْهِمْ رِيحًۭا وَجُنُودًۭا لَّمْ تَرَوْهَا ۚ وَكَانَ ٱللَّهُ بِمَا تَعْمَلُونَ بَصِيرًا ﴿٩﴾
O you who have believed, remember the favor of Allah upon you when armies came to [attack] you and We sent upon them a wind and armies [of angels] you did not see. And ever is Allah, of what you do, Seeing.
Allah's help with wind and unseen armies.
Quran, Surah Al-Ahzaab 33:10
إِذْ جَآءُوكُم مِّن فَوْقِكُمْ وَمِنْ أَسْفَلَ مِنكُمْ وَإِذْ زَاغَتِ ٱلْأَبْصَٰرُ وَبَلَغَتِ ٱلْقُلُوبُ ٱلْحَنَاجِرَ وَتَظُنُّونَ بِٱللَّهِ ٱلظُّنُونَا۠ ﴿١٠﴾
[Remember] when they came at you from above you and from below you, and when eyes shifted [in fear], and hearts reached the throats and you assumed about Allah [various] assumptions.
The fear during the siege.
Quran, Surah Al-Ahzaab 33:22
وَلَمَّا رَءَا ٱلْمُؤْمِنُونَ ٱلْأَحْزَابَ قَالُوا۟ هَٰذَا مَا وَعَدَنَا ٱللَّهُ وَرَسُولُهُۥ وَصَدَقَ ٱللَّهُ وَرَسُولُهُۥ ۚ وَمَا زَادَهُمْ إِلَّآ إِيمَٰنًۭا وَتَسْلِيمًۭا ﴿٢٢﴾
And when the believers saw the companies, they said, "This is what Allah and His Messenger had promised us, and Allah and His Messenger spoke the truth." And it increased them only in faith and acceptance.
The believers' faith grew when they saw the Confederates.
Sahih al-Bukhari 4114
Sahih... وغلب الأحزاب وحده ...
"And He alone defeated the Confederates."
His words of praise after the siege.
Where the ruling is recorded
- Sirat Ibn Hisham, Account of the Battle of the Trench. The coalition, the trench and the end of the siege.
- al-Rahiq al-Makhtum (al-Mubarakpuri), Chapter on the Battle of al-Ahzab. Dates it to Shawwal 5 AH.
- Fath al-Bari (Ibn Hajar), Commentary on Kitab al-Maghazi, chapter on the Battle of the Trench, which is al-Ahzab. Discusses the date and the names of the battle.
This answer is pending review by the scholars on our team. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.