French forces captured Mexico City in 1863 during the intervention that followed the Cinco de Mayo victory.
Mexico defeated a French army at Puebla on May 5, 1862, but the result was a temporary setback rather than the end of the campaign. Napoleon III sent additional troops and renewed the advance. French forces besieged Puebla in 1863, and the city eventually fell after its defenders ran short of supplies.
With the route open, French troops entered Mexico City later in 1863. The occupation allowed France and its Mexican allies to establish a new political order. In 1864, the Austrian archduke Maximilian accepted the imperial crown of the Second Mexican Empire.
The French occupation faced persistent republican resistance led by Benito Juárez. France withdrew its troops in 1866 and 1867, and Maximilian was captured and executed in 1867. Thus, Cinco de Mayo celebrates a successful defense in 1862 even though France later occupied the capital.