Political map of North America & the Caribbean 22 January 1866 (French withdrawal from Mexico): The defeat of the Confederacy (American Civil War) allowed the United States to pressure the French to end their intervention in Mexico (French intervention in Mexico). Napoleon III, already wary of the mounting financial costs and intractable Mexican resistance, declared victory in 1866 and proceeded to withdraw his forces. With its French backers gone, the Mexican Empire (Second Mexican Empire) collapsed over the following year.