Why does Surat al-Tawba not open with the basmala?

Surat al-Tawba, also called "Bara'a", is the only sura of the mushaf at whose head the basmala is not written. In his introduction to the sura in al-Tafsir al-Wasit, Muhammad Sayyid Tantawi sets out the scholars' explanations as al-Qurtubi summarised them, and then states which he relies on. ^{1 2}

• The custom of revoking treaties: it was said that when the pre-Islamic Arabs wished to revoke a treaty they wrote a letter without the basmala; so when Bara'a came down revoking the treaties, the Prophet ﷺ sent Ali ibn Abi Talib, who read it at the pilgrimage season without the basmala, following their custom. ¹
• The report from Uthman: al-Nasa'i reports, and al-Tirmidhi has something similar, through Yazid al-Farisi, that Ibn Abbas asked Uthman why they had joined al-Anfal, one of the mathani, to Bara'a, one of the mi'un, without writing the basmala between them; he answered that al-Anfal was among the first revelations at Medina and Bara'a among the last of the Qur'an, that their stories were alike, and that the Prophet died without making clear that it belonged to it. ¹
• Its opening fell away: Ibn Wahb, Ibn al-Qasim and Ibn Abd al-Hakam report from Malik that when the beginning of the sura fell away the basmala fell away with it; Ibn Ajlan heard that it had been as long as al-Baqara or nearly so, and Sa'id ibn Jubayr said it was like Surat al-Baqara. ¹
• One sura or two: Kharija, Abu Isma and others said that when the mushaf was written under Uthman the Companions differed over whether the two were one sura or two; so a gap was left for those who said two, and the basmala omitted for those who said one. ¹
• The basmala is a safe-conduct: it is reported that Ibn Abbas asked Ali, who said: the basmala is a safe-conduct, and Bara'a came down with the sword and holds no safe-conduct; al-Mubarrad said much the same, that the basmala opens what is good while the start of this sura is a warning and a revoking of treaties. ²

After these views al-Qurtubi said: the correct one is that the basmala was not written because Gabriel did not bring it down in this sura. ² Tantawi says that this is the view he relies on and that careful scholars preferred it, and quotes in support Fakhr al-Din al-Razi, who after listing six explanations said: the correct one is that the Prophet ordered this sura placed after al-Anfal by revelation, and the basmala dropped from its head by revelation. ² He quotes the author of al-Jalalayn that it was not written because the Prophet did not order it, and al-Jamal in his gloss that no one's opinion has any say in including or omitting it, since "the Lawgiver's silence where a statement was due is a statement that it is absent". ²

Tantawi holds that the first and fifth views are reasoned explanations of the wisdom involved, not a basis for stating the cause. ² On the report from Uthman he quotes a verifying scholar — Ahmad Shakir, in his edition of the Musnad of Ahmad — that its chain is "very doubtful", in his judgement "very weak" and indeed "baseless", since it turns on Yazid al-Farisi, about whom it was disputed whether he was Yazid ibn Hurmuz or someone else, whom al-Bukhari listed among weak narrators, and since it casts doubt on what is established by mass transmission about the suras and the basmala at their heads. ³ He rejects the third view as having no evidence or chain and as touching the sanctity of the Qur'an. ³ He judges the fourth weak, because al-Anfal and al-Tawba have been known as separate suras since the Prophet's time and each has its own subject: al-Anfal speaks at length of the battle of Badr, al-Tawba of the expedition to Tabuk in year 9. ³

In Fath al-Qadir, al-Shawkani gives the scholars' views on why the basmala is missing at the head of al-Tawba, among them al-Mubarrad's view of the Arab custom when revoking treaties; the view that some Companions held the two to be one sura, so the basmala was omitted and a gap left to satisfy both parties; and the report of Abu al-Shaykh and Ibn Marduya from Ibn Abbas from Ali that the basmala is a safe-conduct and Bara'a came down with the sword. ^{4 5} He gives the report from Uthman about joining al-Anfal to Bara'a, attributing it to Ibn Abi Shayba, Ahmad, Abu Dawud, al-Tirmidhi — "who graded it hasan" — al-Nasa'i and al-Hakim, "who graded it sahih". ⁵ He thus reports these favourable gradings from al-Tirmidhi and al-Hakim, while Tantawi quotes Ahmad Shakir's judgement that the chain is weak.