Colonel Mercer, the commandant of Oswego, saw at once that the French artillery would, as soon as they opened fire, blow the stockade into pieces, and thinking it better to lose the fort, alone, than the fort and its garrison, he sent boats across the river after nightfall, and the garrison, having spiked their guns, and thrown their ammunition into the well, crossed the river, unperceived by the French.

But Oswego was in no position for defence.  Fort Pepperell stood on the mouth of the river, facing Fort Ontario.  Towards the west and south the place was protected by an outer line of earthworks, mounted with cannon, but the side facing the river was wholly exposed, in the belief that Fort Ontario would prevent any attack in this direction.

Montcalm lost no time.  The next evening, his whole force set to work throwing up a battery, at the edge of the rising ground on which Fort Ontario stood, and, by daybreak, twenty heavy guns were in position, and at once opened fire.  The grape and round shot swept the English position, smashing down the mud-built walls, crashing through the stockades, and carrying destruction among the troops.  The latter made a shelter of pork barrels, three high and three deep, and planted cannon behind them, and returned the enemy’s fire; but the Canadians and Indians had crossed the river, by a ford two miles up, and soon opened fire from all sides.

Colonel Mercer, who had bravely led his men, and inspired them by his example, was cut in two by a cannon shot, and the garrison were seized with despair.  A council of officers was held, and the garrison surrendered as prisoners of war, to the number of sixteen hundred, which included sick, the sailors belonging to the shipping, labourers, and upwards of a hundred women.

Montcalm had the greatest difficulty in preventing the Indians, by means of threats, promises, and presents, from massacring the prisoners.  Oswego was burned to the ground, the forts and vessels on the stocks destroyed, and, the place having been made a desert, the army returned with their prisoners and spoil to Montreal.

The loss of Oswego had inflicted a very severe blow to the influence and prestige of England, among the Indians of the lake districts, but this was partly restored by the failure of the French expedition against William Henry, early in the following spring.

The expedition against Louisbourg, to strengthen which the western frontier had been denuded of troops, proved a failure.  A great delay had taken place at home, in consequence of ministerial changes, and it was not until the 5th of May that fifteen ships of the line and three frigates, under Admiral Holbourne, with 5000 troops on board, sailed from England for Halifax, where Loudon was to meet him with the forces from the colony.  But, while the English fleet had been delaying, the French government had obtained information of its destination, and had sent three French squadrons across the Atlantic to Louisbourg.