Lessons of History: Conquest that created harmony: Last weekend saw the 1,500th anniversary of a miracle whose impact was not only religious, writes John Julius Norwich

THERE IS something curiously threatening about Monte Gargano in Italy; something even a little ominous about the way it rises so sharply and suddenly from the wide plain of northern Apulia - an immense, looming mass of limestone, its darkness emphasised by the little white-painted villages sprinkled across its slopes; shouldering its way some 40 miles out into the Adriatic.

Since the earliest antiquity, an aura of holiness has hung over the mountain. Already in classical times it boasted two famous shrines, one to the ancient warrior-hero Podalcirius, the other - rather more popular - an oracle dedicated to old Calchas, the soothsayer of The Iliad. As recently as the Sixties, I visited the Franciscan monastery at San Giovanni Rotondo to see Padre Pio, Italy's most celebrated living saint, who bore the stigmata and cured diseases by the touch of his mittened hands. But among the countless mystical and miraculous events associated with the history of Monte Gargano, one stands out: the appearance, to Bishop Laurentius of Siponto on 8 May 493 of the great prince of all the angels and leaders of the celestial host, the Archangel Michael.

Three days before, so the story goes, a local farmer set out in search of a fine bull which had wandered away from his herd. He had almost given up hope when he eventually traced it to a dark cave, deep in the mountainside. However, the bull stubbornly resisted all his efforts to entice it out. Finally, in despair, he drew his bow at it - but the arrow stopped dead in mid-flight, made a sharp about-turn and embedded itself deeply into his own thigh. Limping home, he reported the incident to his local bishop; Laurentius ordered a three-day fast throughout the diocese, and on the third day visited the scene of the miracle. Scarcely had he reached the cave when St Michael appeared, armed cap-a-pie (head to foot), and commanded that the cave should henceforth be a shrine to himself and to all his angels. When the bishop returned a few days later, he found the grotto transformed into a chapel, its walls hung with purple; all was bathed in soft, warm light. On the high rock above the entrance, he built the church as instructed; and a little over four months later, on 29 September, dedicated it to the Archangel.

When I returned to the shrine last summer for the first time in 30 years, I found it sadly sanitised. Gone were the votive offerings that once lined the walls of the stairs leading down to the cave - the crutches and the trusses and wooden legs; the eyes and ears and noses stamped on sheets of tin; the primitive poster-paintings of runaway horses, highway collisions, collapsing bridges and overturning saucepans, accidents in which the Archangel had intervened to bring salvation or escape; the fancy-dress costumes worn by small children in his honour, with their tiny wooden swords, silver-foil wings and biscuit-tin breastplates. But, the cave was unchanged. The tremendous pair of Byzantine bronze doors - gift of a rich Amalfitan in 1076 - were still in place; the air within was still loud with devoted mutterings, still damp with the water that drips constantly from the glistening rock and is subsequently dispensed to the faithful in little plastic beakers.

Thus, for exactly 15 centuries, Monte Sant' Angelo has been, with Rome, Canterbury and Compostela in northern Spain, one of the great pilgrim shrines of Europe. It has been consecrated by popes, like Gregory the Great in the sixth century; by saints, like St Francis in the thirteenth - who set a poor example to his followers by carving his initial on the altar just inside the entrance; and by emperors, like the mystic, megalomaniac Saxon Otto III, who, at the very end of the 10th century, walked barefoot from Rome. But it was also visited, on a somewhat humbler level in 1016, by a band of young Norman pilgrims returning from the Holy Land - whose conversation with a curiously-dressed stranger in that very cave led to the foundation of the most magnificent kingdom of medieval Christendom.

The Archangel Michael had always been popular with the Normans, who - like the Italians and the English - had their own mountain jutting out into the sea and bearing his name. Their journey to Jerusalem normally involved riding southward across the Alps and through Italy before finding a ship at Brindisi or Bari; and they often made a point of calling in at Monte Sant' Angelo to give thanks for the dangers past and seek protection in those to come. For this particular group, however, the shrine was to have a special significance: for the stranger who accosted them proved to be a noble Lombard from Bari named Melus, driven into exile after an unsuccessful insurrection against Byzantium, which then held sway over most of south Italy. Lombard independence, he told them, could easily be achieved; all that was needed was the help of a few stalwart young Normans like themselves. Against a combined army, the Greeks would not stand a chance; and when victory was won, his people would not forget their allies.

As for his hearers, they asked nothing better. In the century since their Viking forebears had sailed up the Seine and been formally enfeoffed by the French king, Normandy had seen a population explosion which had produced hundreds of footloose younger sons, all looking for adventure and intent on carving out somehow, somewhere, a territory that they could call their own. They told Melus that they would willingly give him the help he asked. At present, being pilgrims, they were ill-equipped; but they would be back the following year, and the great enterprise would begin.

It was to succeed beyond their wildest hopes. Within little more than half a century the Normans had easily overpowered their Lombard hosts and were masters of virtually all Italy south of Rome. In 1053 their leader Robert Guiscard - the most brilliant military adventurer to have appeared on the European stage between Julius Caesar and Napoleon - defeated a papal army at Civitate, taking prisoner Pope Leo IX; only six years later Pope Nicholas II, accepting the inevitable, invested Robert with the duchies of Apulia, Calabria and Sicily. Admittedly, this last came as a surprise, the Normans having not yet set foot on the island; but they invaded in 1061, and in 1072 captured Palermo. The following century saw the duchy develop into a kingdom - a kingdom which in its tragically short life was to emerge as a cultural phenomenon unique in Europe.

Sicily had originally been part of the Greek world, but in the ninth century had been partly occupied by the Saracens of North Africa. The Normans thus found it divided and hopelessly disorganised. Astonishingly, however, Robert Guiscard's nephew, Roger, and the latter's son, King Roger II, succeeded in uniting the island politically, fusing all that was best of the three great civilisations of the Mediterranean - Western European, Greek and Arab - into a single multiracial, multilingual state. If you would see this almost incredible achievement symbolised in architectural terms, you have only to go to the Palatine Chapel in the Royal Palace of Palermo. Built by Roger II between 1129 and 1140, its ground plan is entirely Latin, as is the sparkling decoration in opus alexandrinum; but the walls are encrusted with Byzantine mosaics as fine as any in Constantinople, and above them is suspended a stalactite ceiling of wood in the classical Islamic style, intricately decorated with the earliest datable series of Arabic paintings in existence anywhere.

The Kingdom of Sicily lasted just 64 years. In a century which saw the great schism between the Eastern and Western Churches and the interconfessional slaughter of the Crusades, it shone forth in bigoted medieval Europe as an object-lesson in tolerance and enlightenment; indeed, it set an example that might profitably be followed in much of the world today. But it could never have come about but for that chance meeting between a single Lombard and a party of pilgrims in that same dark, damp cave once visited by St Michael the Archangel, 1,500 years ago.

John Julius Norwich, the second Viscount Norwich of Aldwick, read French and Russian at Oxford, before joining the Foreign Office in 1952, serving at the embassies in Belgrade and Beirut. In 1964 he resigned to concentrate on writing and is the author of A History of Venice, and Byzantium: The Early Centuries as well as Byzantium: The Apogee. He has also produced books on travel, architecture and Glyndebourne, and the prose and poetry anthologies Christmas Crackers and More Christmas Crackers. He is chairman of the Venice in Peril Fund, and a fellow of the Royal Society of Literature, Royal Geographical Society and Society of Antiquaries.

(Photographs omitted)