The time was favorable for a plan conceived by the emperor to convert Mexico into an empire under a French protectorate.

The principle known as the Monroe Doctrine forbade the establishment of any European power upon the Western hemisphere; but the United States was powerless at the moment to defend it, and by the time her hands were free, even if she were not disrupted, an Empire of Mexico would be established, and French troops could defend it.
In a few months the French army was in the city of Mexico, and an Austrian prince was proclaimed emperor of a Mexican empire.
This ill-conceived expedition came to a tragic and untimely end in 1867.

The civil war ended triumphantly for the Union.

Napoleon, realizing that, with her hands free, the United States would fight for the maintenance of the Monroe Doctrine, promptly withdrew the French army from Mexico, leaving the emperor to his fate.

A republic was at once established, and the unfortunate Maximilian was ordered to be shot.
The finances of France and the prestige of the emperor had both suffered from this miserable attempt.