Chesterton's apologia for the First Crusade

From Ch. 11 of The New Jerusalem:

The Crusader would have been quite justified in suspecting the Moslem even if the Moslem had merely been a new stranger; but as a matter of history he was already an old enemy. The critic of the Crusade talks as if it had sought out some inoffensive tribe or temple in the interior of Thibet, which was never discovered until it was invaded.

They seem entirely to forget that long before the Crusaders had dreamed of riding to Jerusalem, the Moslems had almost ridden into Paris. They seem to forget that if the Crusaders nearly conquered Palestine, it was but a return upon the Moslems who had nearly conquered Europe.
There was no need for them to argue by an appeal to reason, as I have argued above, that a religious division must make a difference; it had already made a difference. The difference stared them in the face in the startling transformation of Roman Barbary and of Roman Spain. In short it was something which must happen in theory and which did happen in practice; all expectation suggested that it would be so and all experience said it was so.

Having thought it out theoretically and experienced it practically, they proceeded to deal with it equally practically. The first division involved every principle of the science of thought; and the last
developments followed out every principle of the science of war. The Crusade was the counter-attack. It was the defensive army taking the offensive in its turn, and driving back the enemy to his base.