Because of the dangers posed by the thirteen rebellious colonies to the south, the British in 1775 erected a third fort on the site of the previous fort. A year before the Declaration of Independence transformed the rebellious colonies into the United States of America, two small armies of rebels invaded the Province of Quebec in an attempt to defeat the British on Canadian soil and to raise Canadian opposition to royal British rule. Fort Saint-Jean played a vital role in the defence of the Province of Quebec against the Americans; indeed, it was the principal defense of Montreal against General Richard Montgomery’s army, which had planned to use the Richelieu River to reach Montreal, then march to Quebec City. The garrison of Fort Saint-Jean fought valiantly against the American troops during a forty-five-day siege before surrendering to the invaders. The siege of Fort Saint-Jean considerably weakened and slowed the American offensive: Montgomery's expedition arrived at Quebec City early in December instead of mid-October, and only three hundred of his men, out of about two thousand, actually reached Quebec City.