Brian DeLay is associate professor of history at the University of California, Berkeley, and the author of "War of a Thousand Deserts: Indian Raids and the U.S.-Mexican War."

Who really won the U.S. Civil War? Mexican president Benito Juárez had a case to make.

Mexico endured its own destructive civil war, between liberals and conservatives, in the late 1850s. Juárez and the liberals triumphed on Dec. 21, 1860, the day after South Carolina seceded from the Union, only to face a French invasion a year and a half later. Colluding with Mexican conservatives, French Emperor Napoleon III drove Juárez across the U.S. border and convinced a compliant Austrian archduke to become Maximilian I, emperor of Mexico.

American money and "decommissioned" guns helped Mexico’s liberals retake the country from Napoleon.

Juárez desperately needed money and war material. He dispatched agents across the U.S. to court skeptical capitalists, plead with arms dealers, inspire or bribe newspaper editors and even establish “Monroe Doctrine Societies” to awaken Americans to their hemispheric responsibilities. It all helped; but not that much. The Union would do nothing to provoke France for fear of European intervention into the Civil War. And amid reports of confederates intercepting guns meant for Mexican liberals, Lincoln imposed a strict ban on all arms exports in late 1862.

But Union victory in 1865 meant a more confrontational policy toward the French and an end to the arms ban. Retired U.S. officers partnered with Juárez to organize volunteers, raise capital and ship huge amounts of newly idled war material south (all for handsome fees). Ulysses S. Grant dispatched troops to the U.S.-Mexican border where they gamely menaced Maximilian’s forces and conveniently left tens of thousands of "decommissioned" guns for Juárez’s men.

The pressure convinced Napoleon to begin withdrawing his troops in 1866. American money and guns helped Mexico’s liberals retake the country step by step, and they finally captured and executed Maximilian in June 1867. Restored to power, Benito Juárez and his successors consolidated a sweeping program of economic, social and institutional reform that would profoundly shape his country’s future.