American President Abraham Lincoln had supported the…

American President Abraham Lincoln had supported the republicans under Juárez, but was unable to intervene due to the American Civil War.

Immediately after the end of the war, in 1865, United States Army General Philip Sheridan, under the supervision of President Andrew Johnson and General Ulysses S. Grant, had assembled fifty thousand troops, and dispatched them to the border between Mexico and the United States.

Here, his corps run patrols to visibly threaten intervention against the French, and also supply weapons to Juárez's forces.

The United States Congress had unanimously passed a resolution which opposed the establishment of the Mexican monarchy on April 4, 1864.

On February 12, 1866, in accordance with the Monroe Doctrine, the U.S. requests the French withdraw their forces from Mexico, while moving soldiers to positions along the Rio Grande, and setting up a naval blockade to prevent French reinforcements from landing.

Napoleon III, choosing Franco-American relations over his Mexican monarchy ambitions, will soon announce  the withdrawal of French forces beginning May 31.