It can fairly be said that, until its steep military decline in the nineteenth and twentieth centuries, Islam was the arch-enemy of European civilization. For virtually all of Europe’s history since the Dark Ages it had been a mortal threat. Between the seventh and ninth centuries, militarized Islam conquered half of the fragmented Roman Empire. ‘It very nearly destroyed us,’ wrote Hilaire Belloc in 1938. At the time, Belloc thought it dangerous that Westerners

have forgotten all about Islam. They have never come in contact with it. They take for granted that it is decaying, and that, anyway, it is just a foreign religion which will not concern them. It is, as a fact, the most formidable and persistent enemy which our civilization has had, and may at any moment become as large a menace in the future as it has been in the past…It has always seemed to me possible, and even probable, that there would be a resurrection of Islam and that our sons or our grandsons would see the renewal of that tremendous struggle between the Christian culture and what has been for more than a thousand years its greatest opponent.

Belloc’s language was blunt even for the time. He referred to Western culture as 'white civilization.’ He was an especially ardent defender of the Crusades, which he called the 'one supreme attempt to relieve that [Islamic] pressure upon the Christian West.’ […]

According to the French historian Henri Pirenne, the Islamic conquests created Europe—at least Europe as it has existed since the end of the Roman Empire. Unlike the invasions of Rome by Germanic barbarians in the first centuries after Christ, which were easily absorbed into existing institutions, the Islamic invasions changed everything. Islam’s advance broke the ancient world because it broke the unity of the Mediterranean. Cut off from the Christian capital (and the emperor’s fleet) at Constantinople, Europeans abandoned the Mediterranean to Muslim navies and Saracen pirates. 'The West was bottled up and forced to live by its own means, in a vacuum,’ Pirenne wrote. 'For the first time ever, the center of life was pushed back from the Mediterranean, towards the north.’ Europe’s heart moved away from its southern littoral to somewhere between the Seine and the Rhine. 'Now on the coasts of Mare Nostrum’ Pirenne wrote, 'stretched two different and hostile civilizations.’