...Lewis certainly does understand the Crusades as part of a counter-attack or counter-offensive: a Christian attempt to retake those lands that had been lost to Christendom in the waves of Islamic conquest that began in the seventh (not eighth) century. This isn't a new theme in his writings, but he articulated it best in his Tanner Lectures on "Europe and Islam" (here), delivered in Oxford in 1990:

In recent years it has become the practice, in both western Europe and the Middle East, to see and present the Crusades as an early exercise in Western imperialism--as a wanton and predatory aggression by the European powers of the time against the Muslim or, as some would now say, against the Arab lands.

They were not seen in that light at the time, either by Christians or by Muslims. For contemporary Christians, the Crusades were religious wars, the purpose of which was to recover the lost lands of Christendom and particularly the holy land where Christ had lived, taught, and died. In this connection, it may be recalled that when the Crusaders arrived in the Levant not much more than four centuries had passed since the Arab Muslim conquerors had wrested these lands from Christendom--less than half the time from the Crusades to the present day--and that a substantial proportion of the population of these lands, perhaps even a majority, was still Christian.

Lewis isn't really interested in whether the Crusaders were more or less "awful" or "terrible" or "wrong" than other conquerors, ancient, medieval or modern. Any hack propagandist, movie maker, or Slate journalist can do that, for people who enjoy moralizing across millennia. Lewis instead seeks to instruct us, from the sources, as to how the Crusades were viewed by their contemporaries. Christians at the time saw them as a reconquest of their own lands, not as an imperialist intrusion into Islam's privileged domain. (And Lewis goes on to note that Muslims didn't see the Crusaders as much more than a nuisance, until they began to raid closer to their truly privileged domain, Mecca and Medina.)...