Complex sentence: In Upper Canada (Ontario) the Reform movement was formed in opposition to the Family Compact and was led initially by William Lyon Mackenzie who went on to become the principal figure in the Upper Canada Rebellion of 1837.
Split into simpler sentences: In Upper Canada (Ontario), the Reform movement was formed in opposition to the Family Compact. It was led initially by William Lyon Mackenzie, who became the principal figure in the Upper Canada Rebellion of 1837.