Complex sentence: British General Guy Carleton sent a force composed mostly of Quebec militia in response to news of Allen's crossing of the Saint Lawrence River, which cut off Allen's escape route, and eventually captured Allen and a number of his men.
Split into simpler sentences: British General Guy Carleton sent a force composed mostly of Quebec militia in response to news of Allen's crossing of the Saint Lawrence River. This force off Allen's escape route, and eventually surrounded and captured Allen and a number of his men.