All being now prepared, Cortéz planned his attack. It will be recollected that the city of Mexico rose, like Venice, from the bosom of the placid waters, and that its communication with the main land was kept up by the great causeways which were described in the earlier portion of this narrative. The object of the conqueror, therefore, was to shut up the capital, and cut off all access to the country by an efficient blockade of the lake, with his brigantines, and of the land with his infantry and cavalry. Accordingly he distributed his forces into three bodies or separate camps. The first of these, under Pedro de Alvarado, consisting of thirty horse, one hundred and sixty-eight Spanish infantry, and twenty-five thousand Tlascalans, was to command the causeway of Tacuba. The second division, of equal magnitude, under Olid, was to be posted at Cojohuacan, so as to command the causeways that led eastwardly into the city. The third equal corps of the Spanish army was entrusted to Sandoval, but its Indian force was to be drawn from native allies at Chalco. Alvarado and Olid were to proceed around the northern head of the lake of Tezcoco, whilst Sandoval, supported by Cortéz with the brigantines, passed around the southern portion of it, to complete the destruction of the town of Iztapalapan, which was deemed by the conqueror altogether too important a point to be left in the rear. In the latter part of May, 1521, all these cavaliers got into their assigned military positions, and it is from this period that the commencement of the siege of Mexico is dated, although Alvarado had previously had some conflicts with the people on the causeway that led to his head quarters in Tacuba, and had already destroyed the pipes that fed the water-tanks and fountains of the capital. At length Cortéz set sail with his flotilla in order to sustain Sandoval's march to Iztapalapan. As he passed across the lake and under the shadow of the "rock of the Marquis," he descried from his brigantines several hundred canoes of the Mexicans filled with soldiers and advancing rapidly over the calm lake. There was no wind to swell his sails or give him command of his vessels' motion, and the conqueror was obliged to await the arrival of the canoes without making such disposition for action as was needful in the emergency. But as the Indian squadron approached, a breeze suddenly sprang up, and Cortéz, widening his line of battle, bore down upon the frail skiffs, overturning, crushing and sinking them by the first blow of his formidable prows, whilst he fired to the right and left amid the discomfitted flotilla. But few of these Indian boats returned to the canals of the city, and this signal victory made Cortéz, forever after, the undisputed master of the lake.