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.