After the successful siege of Oswego in August 1756, Governor Vaudrueil sent Montcalm down the Richelieu River to Fort Carillon (Ticonderoga). Vaudrueil correctly assumed that the British were planning to mount an offensive from Fort William Henry at the south end of Lake Champlain. Montcalm commanded a force of 8,000 French regulars, Troupes de la Marine, Canadian militia, and Native allies. He deployed his troops at Fort Carillon, but British General Loudon failed to mount the anticipated attack.
The following year, on 30 June 1757, Montcalm began to advance on Fort William Henry. The British managed to send 1,800 reinforcements to the fort in July, increasing the garrison to 2,400 troops. In late July, the Canadian militia and Native warriors had isolated the fort, while Montcalm established artillery positions and deployed his regulars. By 2 August, French artillery was bombarding the fort, while the militia and Native allies ranged through the surrounding woods. After six days, the commander of the garrison, Colonel Monro, surrendered Fort William Henry to Montcalm and his troops. Montcalm extended the courtesies of European warfare to Colonel Munro, granting the British safe passage back to New York on the condition that they took no further part in hostilities.
This arrangement astounded the Canadian militia, and the Native warriors were furious. On 9 August 1757, as the British troops marched out of Fort William Henry, the Native warriors and some of the militia fell upon them. Enraged, Montcalm personally led French regulars into the melee, confronting the Native warriors and the militia. Before Montcalm could restore order, however, 150 British soldiers perished in the onslaught.
The news of the events at Fort William Henry reached New York, and the British and Americans were horrified. The British government became even more determined to end the French "menace" in Canada. The British prime minister, William Pitt, dispatched General James Abercromby to America with reinforcements. They arrived in New York on 30 December 1757. On 19 February 1758, General Jeffrey Amherst arrived with additional troops for an assault on Louisbourg.