The American Continental Congress authorized military operations against Canada on 27 June 1775. By August, the Americans had assembled forces for the invasion of Canada. General Schuyler was to command the army that advanced up the Richelieu River to Montreal, but he fell ill and was replaced by General Richard Montgomery. A second force, led by Major-General Benedict Arnold, moved up the Kennebec and Chaudière rivers towards Quebec.
The American force entered Montreal unopposed on 13 November. Sir Guy Carleton, the Governor of Quebec, had ordered the Montreal garrison to Quebec. By 4 December 1775, General Montgomery had marched from Montréal to Ste.-Foy and joined up with Arnold's force. The Quebec garrison consisted of 70 Royal Fusiliers, 230 soldiers of Lieutenant-Colonel Allan McLean's Royal Highland Immigrants, 330 British (British colonists) militia, 480 Royal Navy sailors and Marines, and 543 Canadian militia commanded by Colonel Noel Voyer.
By 4 December 1775, General Montgomery reached Ste.-Foy and joined up with Major-General Arnold's force. Montgomery launched an assault on Quebec in a blinding snowstorm at 4:00 a.m. on 31 December 1775. The British and Canadians easily repulsed the Americans. General Montgomery died in the initial fighting, and Arnold assumed command of the American force.
The siege continued, and the American Congress sent additional troops to reinforce the American force outside Quebec. In March, 2,500 troops joined Arnold's force. In April, an additional 2,500 American soldiers arrived led by Major-General John Thomas, who took command of the entire army. On 6 May 1776, as the Americans prepared to renew the attacks on Quebec, British ships arrived with much-needed supplies and troops for the beleaguered Quebec garrison. Seven battalions of British regulars under the command of Major-General John Burgoyne marched into Quebec.
General Thomas was faced with the prospect of meeting the British and Canadians on equal terms. He immediately lifted the siege and the American force hastily withdrew to Montreal.