And it was in that moment of distress and confusion that the whip of terror laid its most nicely calculated lash about his heart. It dropped with deadly effect upon the sorest spot of all, completely unnerving him. He had been secretly dreading all the time that it would come - and come it did.Far overhead, muted by great height and distance, strangely thinned and wailing, he heard the crying voice of Defago, the guide.The sound dropped upon him out of that still, wintry sky with an effect of dismay and terror unsurpassed. The rifle fell to his feet. He stood motionless an instant, listening as it were with his whole body, then staggered back against the nearest tree for support, disorganized hopelessly in mind and spirit. To him, in that moment, it seemed the most shattering and dislocating experience he had ever known, so that his heart emptied itself of all feeling whatsoever as by a sudden draught.'Oh! oh! This fiery height! Oh, my feet of fire! My burning feet of fire...' ran in far, beseeching accents of indescribable appeal this voice of anguish down the sky. Once it called - then silence through all the listening wilderness of trees.And Simpson, scarcely knowing what he did, presently found himself running wildly to and fro, searching, calling, tripping over roots and boulders, and flinging himself in a frenzy of undirected pursuit after the Caller. Behind the screen of memory and emotion with which experience veils events, he plunged, distracted and half-deranged, picking up false lights like a ship at sea, terror in his eyes and heart and soul. For the Panic of the Wilderness had called to him in that far voice - the Power of untamed Distance - the Enticement of the Desolation that destroys. He knew in that moment all the pains of someone hopelessly and irretrievably lost, suffering the lust and travail of a soul in the final Loneliness. A vision of Defago, eternally hunted, driven and pursued across the skyey vastness of those ancient forests fled like a flame across the dark ruin of his thoughts...It seemed ages before he could find anything in the chaos of his disorganized sensations to which he could anchor himself steady for a moment, and think...The cry was not repeated; his own hoarse calling brought no response; the inscrutable forces of the Wild had summoned their victim beyond recall - and held him fast.("The Wendigo")
What happened? It took Gibbon six volumes to describe the decline and fall of the Roman Empire, so I shan’t embark on that. But thinking about this almost incredible episode does tell one something about the nature of civilisation. It shows that however complex and solid it seems, it is actually quite fragile. It can be destroyed. 

What are its enemies?

Well, first of all fear — fear of war, fear of invasion, fear of plague and famine, that make it simply not worthwhile constructing things, or planting trees or even planning next year’s crops. And fear of the supernatural, which means that you daren’t question anything or change anything. The late antique world was full of meaningless rituals, mystery religions, that destroyed self-confidence. And then exhaustion, the feeling of hopelessness which can overtake people even with a high degree of material prosperity. 

There is a poem by the modern Greek poet, Cavafy, in which he imagines the people of an antique town like Alexandria waiting every day for the barbarians to come and sack the city. Finally the barbarians move off somewhere else and the city is saved; but the people are disappointed — it would have been better than nothing. Of course, civilisation requires a modicum of material prosperity—

What civilization needs:

confidence in the society in which one lives, belief in its philosophy, belief in its laws, and confidence in one’s own mental powers. The way in which the stones of the Pont du Gard are laid is not only a triumph of technical skill, but shows a vigorous belief in law and discipline. Vigour, energy, vitality: all the civilisations—or civilising epochs—have had a weight of energy behind them. People sometimes think that civilisation consists in fine sensibilities and good conversations and all that. These can be among the agreeable results of civilisation, but they are not what make a civilisation, and a society can have these amenities and yet be dead and rigid.