By dethroning Theodosius III. on the very eve of the great Saracen
 invasion, Leo the Isaurian took upon himself the gravest of
 responsibilities. With a demoralized army, which of late had been more
 accustomed to revolt than to fight, a depleted treasury, and a
 disorganized civil service, he had to face an attack even more dangerous
 than that which Constantine IV. had beaten off thirty years before.
 Constantine too, the fourth of a race of hereditary rulers, had a secure
 throne and a loyal army, while Leo was a mere adventurer who had seized
 the crown only a few months before he was put to the test of the sword.
 
 The reigning Caliph was now Suleiman, the seventh of the house of the
 Ommeyades. He had strained all the resources of his wide empire to provide
 a fleet and army adequate to the great enterprise which he had taken in
 hand. The chief command of the expedition was given to his brother
 Moslemah, who led an army of eighty thousand men from Tarsus across the
 centre of Asia Minor, and marched on the Hellespont, taking the strong
 city of Pergamus on his way. Meanwhile a fleet of eighteen hundred sail
 under the vizier Suleiman, namesake of his master the Caliph, sailed from
 Syria for the Aegean, carrying a force no less than that which marched by
 land. Fleet and army met at Abydos on the Hellespont without mishap, for
 Leo had drawn back all his resources, naval and military, to guard his
 capital.
 
 In August, 717, only five months after his coronation, the Isaurian saw
 the vessels of the Saracens sailing up the Propontis, while their army had
 crossed into Thrace and was approaching the city from the western side.
 Moslemah caused his troops to build a line of circumvallation from the sea
 to the Golden Horn, cutting Constantinople off from all communication with
 Thrace, while Suleiman blocked the southern exit of the Bosphorus, and
 tried to close it on the northern side also, so as to prevent any supplies
 coming by water from the Euxine. Leo, however, sallied forth from the
 Golden Horn with his galleys and fire-vessels bearing the dreaded Greek
 fire, and did so much harm to the detachment of Saracen ships which had
 gone northward up the strait, that the blockade was never properly
 established on that side.
 
 The Saracens relied more on starving out the city than on taking it by
 storm: they had come provided with everything necessary for a blockade of
 many months, and sat down as if intending to remain before the walls for
 an indefinite time. But Constantinople had been provisioned on an even
 more lavish scale; each family had been bidden to lay in a stock of corn
 for no less a period than two years, and famine appeared in the camp of
 the besiegers long ere it was felt in the houses of the besieged. Nor had
 Moslemah and Suleiman reckoned with the climate. Hard winters occasionally
 occur by the Black Sea, as our own army learnt to its cost in the Crimean
 War. But the Saracens were served even worse by the winter of 717-18, when
 the frost never ceased for twelve weeks. Leo might have boasted, like Czar
 Nicholas, that December, January, and February were his best generals--for
 these months wrought fearful havoc in the Saracen host. The lightly clad
 Orientals could not stand the weather, and died off like flies of
 dysentery and cold. The vizier Suleiman was among those who perished.
 Meanwhile the Byzantines suffered little, being covered by roofs all the
 winter.
 
 When next spring came round Moslemah would have had to raise the siege if
 he had not been heavily reinforced both by sea and land. A fleet of
 reserve arrived from Egypt, and a large army came up from Tarsus and
 occupied the Asiatic shores of the Bosphorus.
 
 But Leo did not despair, and took the offensive in the summer. His
 fire-ships stole out and burnt the Egyptian squadron as it lay at anchor.
 A body of troops landing on the Bithynian coast, surprised and cut to
 pieces the Saracen army which watched the other side of the strait. Soon,
 too, famine began to assail the enemy; their stores of provisions were now
 giving out, and they had harried the neighbourhood so fiercely that no
 more food could be got from near at hand, while if they sent foraging
 parties too far from their lines they were cut off by the peasantry. At
 last Moslemah suffered a disaster which compelled him to abandon his task.
 The Bulgarians came down over the Balkans, and routed the covering army
 which observed Adrianople and protected the siege on the western side. No
 less than twenty thousand Saracens fell, by the testimony of the Arab
 historians themselves, and the survivors were so cowed that Moslemah gave
 the order to retire. The fleet ferried the land army back into Asia, and
 both forces started homeward. Moslemah got back to Tarsus with only thirty
 thousand men at his back, out of more than a hundred thousand who had
 started with him or come to him as reinforcements. The fleet fared even
 worse: it was caught by a tempest in the Aegean, and so fearfully
 shattered that it is said that only five vessels out of the whole Armada
 got back to Syria unharmed.
 
 Thus ended the last great endeavour of the Saracen to destroy
 Constantinople. The task was never essayed again, though for three hundred
 and fifty years more wars were constantly breaking out between the Emperor
 and the Caliph. In the future they were always to be border struggles, not
 desperate attempts to strike at the heart of the empire, and conquer
 Europe for Islam. To Leo, far more than to his contemporary the Frank
 Charles Martel, is the delivery of Christendom from the Moslem danger to
 be attributed. Charles turned back a plundering horde sent out from an
 outlying province of the Caliphate. Leo repulsed the grand-army of the
 Saracens, raised from the whole of their eastern realms, and commanded by
 the brother of their monarch. Such a defeat was well calculated to impress
 on their fatalistic minds the idea that Constantinople was not destined by
 providence to fall into their hands. They were by this time far removed
 from the frantic fanaticism which had inspired their grandfathers, and the
 crushing disaster they had now sustained deterred them from any repetition
 of the attempt. Life and power had grown so pleasant to them that
 martyrdom was no longer an "end in itself"; they preferred, if checked, to
 live and fight another day.
 
 Leo was, however, by no means entirely freed from the Saracens by his
 victory of 718. At several epochs in the latter part of his reign he was
 troubled by invasions of his border provinces. None of them, however, were
 really dangerous, and after a victory won over the main army of the
 raiders in 739 at Acroinon in Phrygia, Asia Minor was finally freed from
 their presence.