Final answer:
Mexicans successfully expelled all French troops in 1867 after a prolonged period of conflict and with support from the United States after its Civil War.
Step-by-step explanation:
The year in which Mexicans finally expelled all French troops was 1867. The French intervention in Mexico started when a large French fleet took Veracruz in 1861, seeking to establish a French empire in the Americas while the United States was preoccupied with the Civil War.
Despite an initial setback for the French with their defeat by Mexican forces in 1862, France managed to establish a regime in Mexico. However, the French presence was not to last.
After several years of struggle, the French-supported regime collapsed, and with the support of the United States post-Civil War, Mexicans succeeded in defeating the French forces, resulting in the French withdrawal.