As I said, the more pertinent question is, even if there is a rough historicity, why do you believe that it happened in anything like the manner you take as highly accurate given it's quite obviously not recorded as factual history and serves a theological purpose.
So, regarding the sources that record the event written by a secular historian:
The contrast can be seen more clearly when specific examples are analyzed in their larger context, such as the two authors' accounts concerning the raid on the B. Qaynuqac, the exile of the B. Nadir, and the raid on the B. Qurayza. These events constitute a unit within the structural framework of the maghazi and indicate Muhammad's relations with the Medinan Jews-in mythical terms, the hero's journey away from home to prove himself.66 Although these tribes were not the only Jews in Medina, they were certainly the most significant, and Muhammad is depicted as having been responsible for bringing about their destruction. The way this happened is explained differently by Ibn Ishaq and al-Waqid
Ibn Ishaq represents Muhammad's conflict with the Jews of Medina in a truly eclectic fashion: with the B. Qaynuqac, we have the Prophet inviting the Jews to Islam in typical Biblo-Qur'anic manner; with the B. Nadir, Ibn Ishaq introduces instead the universal mythical pattern of stone-throwing, for we see the B. Nadir plan to drop a rock upon Muhammad in order to kill him;67 and with the B. Qurayza, we see borrowing from the tales of the ayyam, in particular a story which told of how the B. Qurayza had been massacred by Malik ibn Ajlan in the days of the Jahiliya.68 A story similar to this last example is also related regarding the Christians of Naj- ran, who were said to have been massacred according to some pre-Islamic tradi- tions cited by Ibn Ishaq.69 In his depiction of the actual destruction of the tribes, Ibn Ishaq uses a combination of mnemonic and Biblo-Qur'anic patterns: the community that rejects Muhammad is obliterated in so decisive a fashion that not only are the better-prepared Jews defeated by the smaller Muslim forces, but none of the Jewish tribes is ever heard of again. As for the actual means of Muhammad's victory, the violence against the Jews is depicted as having escalated from forced submission to exile and execution.
Al-Waqidi, for his part, plays with Ibn Ishaq's account, using repetition, a change of chronology, and new material (as is his wont) to weave a motif about the Jews' abrogation of the agreement with Muhammad. This, too, is an age-old biblical theme: the Jews had not kept their covenant with God. But al-Waqidi does not stop here. He takes aspects of the B. Nadir incident depicted by Ibn Ishaq and presents them during the raid on the B. Qaynuqac as well, so that the hypocrisy of Ibn Ubay is repeated, as is the notion of the exile of the Jews. Through repetition al-Waqid emphasizes that the Prophet is honest by character; he is a man who keeps his agreements but is forced to attack the Jews because they have abrogated theirs. As for the Jews, they are portrayed as predictably unfaithful. By emphasizing the writing of agreement with the significant Jewish communities, al-Waqidi introduces his own interpretation of these events. A close comparison of the texts of Ibn Ishaq a al-Waqidi is necessary to appreciate more fully the contrived nature of this art form...
In the works of both compilers, the chronology of events is artificial and imposed; it is based on the purposes of the compiler and the interpretation that he desires to impose on this material rather than on a search for factual data.
Muhammad and the Medinan Jews: A Comparison of the Texts of Ibn Ishaq's Kitab Sirat Rasul Allah with al-Waqidi's Kitab al-Maghazi - Rizwi S. Faizer
So, as a non-Muslim, why do you take what is clearly a theological narrative as highly accurate historical fact?