As anyone can see from our debate, historical evidence and Muslim sources mean virtually nothing to Shadid. Nevertheless, for those Muslims who respect their greatest scholars more than their own Westernized reinterpretations, I offer Ibn Kathir's account of what started the Battle of Tabuk. The following is taken from Ibn Kathir's Al-Bidayyah wan-Nihayyah (translated by Wa'il Abdul Mut'aal Shihab as The Battles of the Prophet).
Allah, Most High, ordered the believers to prohibit the disbelievers from entering or coming near the sacred Mosque. On that, Quraish thought that this would reduce their profits from trade. Therefore, Allah, Most High, compensated them and ordered them to fight the people of the Book until they embrace Islam or pay the Jizyah. Allah says,
"O ye who believe! Truly the pagans are unclean; so let them not, after this year of theirs, approach the sacred Mosque. And if ye fear poverty, soon will Allah enrich you, if He wills, out of His bounty, for Allah is All-knowing, All-Wise. Fight those who believe not in Allah nor the Last Day, nor hold that forbidden which hath been forbidden by Allah and His Messenger, nor acknowledge the religion of Truth, from among the People of the Book, until they pay the Jizyah with willing submission, and feel themselves subdued." (Surah 9:28-29)
Therefore, the Messenger of Allah (peace and blessings of Allah be upon him) decided to fight the Romans in order to call them to Islam. (Ibn Kathir, The Battles of the Prophet, pp. 183-4)
Notice that there's absolutely nothing here about any Romans attacking the Muslims. The chain of events leading up to the Battle of Tabuk was:
(1) Muhammad prohibited non-Muslims from taking religious pilgrimages to Mecca. (Strange, isn't it, that Muslims condemned the polytheists when the polytheists prohibited Muslims from taking the pilgrimage to Mecca. Hypocrisy is rampant in early Islam.)
(2) The Quraish (now Muslims) were worried that this would interfere with their profits.
(3) Muhammad received a revelation, saying, in effect, "If you're worried about money, don't worry, because God's going to enrich you by sending you to fight the People of the Book, until they convert to Islam or pay the Jizya!"
Note: If the Roman Empire had converted to Islam, Mecca would have tons of pilgrims visiting the Ka'ba. If the Romans elected to pay the Jizya instead, Muslims would still be raking in tons of money.
Ibn Sa'd adds that word reached Muhammad "that Haraclius had disbursed one year's salary to his soldiers" stationed in Syria (Ibn Sa'd, Kitab al-Tabaqat al-Kabir, p. 204). An army with a year's salary is an attractive target for a religion that funds itself by raids. Indeed, Ibn Ishaq titles his discussion on this topic "the Raid on Tabuk," rather than, say, "The Muslims defend themselves from the Romans."
It seems, then, that the motive for Allah's command in Surah 9:29 was simply to fight people until they started sending money to the Muslims. The early Muslims understood this. Islam's greatest commentators understood this. The only people who don't understand this are Muslims like Shadid Lewis, who view Islam through the lenses of Western Christian tolerance.
For more on the fighting verses of Surah 9, see Sam Shamoun's article here.