Having takes their resolution, the confederate generals began their march in the beginning of September, and on the 6th of that month, both places were invested. Aire, which is comparatively of small extent, was garrisoned by only five thousand seven hundred men; but Venant was a place of great size and strength, and had a garrison of fourteen battalions of foot and three regiments of dragoons, mustering eight thousand combatants. They were under the command of the Count de Guebriant, a brave and skillful commander. Both were protected by inundations, which retarded extremely the operations of the besiegers, the more especially as the autumnal rains had early set in this year with more than usual severity. While anxiously awaiting the cessation of this obstacle, and the arrival of a great convoy of heavy cannon and ammunition which was coming up from Ghent, the Allied generals received the disheartening intelligence of the total defeat of this important convoy, which, though guarded by sixteen hundred men, was attacked and destroyed by a French corps on the 19th September. This loss affected Marlborough the more sensibly, that it was the first disaster of moment which had befallen him during nine years of incessant warfare.[16] But, notwithstanding this disaster, St Venant was so severely pressed by the fire of the besiegers, under the Prince of Anhalt, who conducted the operations with uncommon vigour and ability, that it was compelled to capitulate on the 29th, on condition of its garrison being conducted to St Omer, not to serve again till regularly exchanged. Aire still held out, as the loss of the convoy from Ghent, and the dreadful rains which fell almost without intermission during the whole of October, rendered the progress of the siege almost impossible. The garrison, too, under the command of the brave governor, made a most resolute defence. Sickness prevailed to a great extent in the Allied army; the troops were for the most part up to the knees in mud and water; and the rains, which fell night and day without intermission, precluded the possibility of finding a dry place for their lodging. It was absolutely necessary, however, to continue the siege; for, independent of the credit of the army being staked on its success, it had become impossible, as Marlborough himself said, to draw the cannon from the trenches.[17] The perseverance of the Allied commanders was at length rewarded by success. On the 12th November the fortress capitulated, and the garrison, still three thousand six hundred and twenty-eight strong, marched out prisoners, leaving sixteen hundred sick and wounded in the town. This conquest, which concluded the campaign, was, however, dearly purchased by the loss of nearly seven thousand men killed and wounded in the Allied ranks, exclusive of the sick, who, amidst those pestilential marshes, had now swelled to double the number.[18]