Complex sentence: The American failure to occupy any significant part of Canada prevented them from annexing it for the second reason, which was largely ended by the Era of Good Feelings, which ensued after the war between Britain and the United States.
Split into simpler sentences: The American failure to occupy any significant part of Canada prevented them from annexing it for the second reason, which was largely ended by the Era of Good Feelings. This occurred as a result after the war between Britain and the United States.