In the latter days of the Rebellions of 1837 in Upper Canada, after Mackenzie and 200 of his followers retreated from Toronto to Navy Island, he declared a separate republic.

[6] On December 29, Royal Navy Commander Andrew Drew and seven boatloads of Canadian militiamen crossed the Niagara River to Fort Schlosser.

It was falsely reported that dozens of Americans were killed as they were trapped on board, and U.S. soldiers retaliated by burning a British steamer while it was in U.S. waters, triggering what became known as the Caroline affair.

He and his force retreated to Buffalo, New York, where they were captured by the U.S. army and sentenced in the U.S. to 18 months' imprisonment for violating neutrality laws between the United States and the United Kingdom, ending the prospect of a Canadian declaration of independence/secession and ending what the British authorities described as an inconsequential and unsupported colonial rebellion.

[8] On 16 September 1838, a convention attended by 160 delegates of the organization was held in Cleveland,[9] during which it elected Abram D. Smith the first president of the Republic of Canada.