Fort Saint-Jean is a fort in the Canadian province of Quebec located on the Richelieu River. The fort was first built in 1666 by soldiers of the Carignan-Salières Regiment and was part of a series of forts built along the Richelieu River. Over the years, it was destroyed and rebuilt several times, but it is, after Quebec City, the military site that has been occupied non-stop for the longest time in Canada. The fort is designated as a National Historic Site of Canada, and it currently houses the Royal Military College of Saint-Jean. The fort has been continually occupied since 1748, and is the core from which the city of Saint-Jean-sur-Richelieu, Quebec grew around. Fort Saint-Jean played a crucial role during the 1775 American invasion of the Province of Quebec.