Fort William Henry held the southern end of Lake George in 1757 in New York for the British. This was the southern terminus of the chain of lakes and rivers that provided an invasion route for the British to Quebec and for the French deep into New York.
The fort was constructed by the British in 1755. General Montcalm, commander of the French forces in New France, decided to take Fort William Henry in 1757 to prevent it being used as a springboard for an invasion of Canada. Leading a force of 3000 regulars, 3000 militia and 2000 Indians, Montcalm began the siege on August 3, 1757.
Colonel George Monro commanded the garrison of the Fort, with a mixed force of 2600 regulars and militia. The garrison surrendered on August 9. The British were allowed to retain their muskets, although they were to have no ammunition. The garrison, together with all civilians in the fort, were to be allowed to march to British held Fort Edward. The terms of the agreement specified that the British were to be protected by the French from the Indians. Montcalm talked to the Indians before the surrender to make certain they understood the terms that had been granted to the British.
As the British began their march, the Indians attacked them. Montcalm and other French officers did their best to protect the British. Other French officers pointedly refused to offer any aid to the unarmed British. Estimates of British slain that day vary wildly from 200-1500 with most historians today coming down on the low end of the scale. The Indians took many of the British captive. Montcalm succeeded in ransoming 500 of them and turning them over to the British garrison at Fort Edward on August 15, 1757. The massacre hardened British and American attitudes towards the French and the Indians, and convinced them that a more vigorous prosecution of the war was essential.