Ibn Khaldun's theory was that every urban civilisation becomes vulnerable when it grows decadent from within. People live in towns and get used to luxuries. The rich grow indolent, the poor resentful. There is a loss of asabiyah, a keyword for Khaldun. Nowadays we would probably translate it as "social cohesion". People no longer think in terms of the common good. They are no longer willing to make sacrifices for one another. Essentially they lose the will to defend themselves. They then become easy prey for the desert dwellers, the people used to fighting to stay alive.
Al-Qaeda's application of this principle -- especially after the fall of Soviet Communism during Russia's occupation of Afghanistan -- was not to try to defeat the immoral West on western lands, but to provoke America to come and, like Russia, be mired in defeat in Afghanistan.
Rabbi Sacks says that in the West we focus on Islamic extremism as the primary threat, when the real threat is the loss of our own values and cohesion, which makes our defeat almost certain.
It is a peculiarity of the Abrahamic monotheisms that they see, at the heart of society, the idea of covenant. Covenantal politics are politics with a purpose, driven by high ideals, among them the sanctity of life, the dignity of the individual, the rule of justice and compassion, and concern for the poor, the widow, the orphan and the stranger. G.K. Chesterton called America a "nation with the soul of a church". Britain used to be like that too. In the 1950s there was no television at certain hours on Sunday so as not to deter churchgoing. Sundays helped keep families together, families helped keep communities together, and communities helped keep society together. I, a Jew growing up in a Christian nation, did not feel threatened by this. I felt supported by it — much more than I do now in an ostensibly more tolerant but actually far more abrasive, rude and aggressive society.
He ends with this admonition...
None of us should be in any doubt as to the seriousness of what is at stake. Europe today is pursuing the chimera of societies without a shared moral code, nations without a collective identity, cultures without a respect for tradition, groups without a concern for the common good, and politics without the slightest sense of history. Ibn Khaldun, were he alive, would tell them precisely where that leads. The question is not radical Islam but, does the West believe in itself any more?