The first military confrontation between the Arab and Christian world took place in 674-678. The Caliphate had mobilized all its forces to bring defeat to Constantinople, the capital of the East Roman Empire. The victory of Constantinople saved Europe from Arab domination. The other Christian victory, in 732 in France, was less decisive, since the Caliphate had mobilized only limited forces.
The military confrontation in 674 is also remarkable for other reasons. It was only forty-six years after the flight of Mohammed from Mecca. The caliph, Moawiyah, who undertook the venture, regarded the conquest of Constantinople a holy expedition.
The troops of the Caliph besieged the city by sea and by land. The solid and lofty walls hold out. The secret of artificial or Greek fire, the first chemical warfare, made the decisive difference.
Greek engineers had invented a fluid, made from oil, although up to today the substance is unknown. It burnt on the surface of the water and was released by small Greek vessels just in front of the hostile ships.
It was an inferno and the losses were tremendous. The composition of the Greek fire, which was guarded like Fort Knox, was lost in 1453 and never discovered. (Source: E. Gibbon, The Decline and Fall of the Roman Empire, Volume 5 ( London 1994).