The years 1861-65 were critical ones in the United States, with the country divided and fighting a bloody Civil War.
Most students of U.S. history have a pretty good understanding of the issues at the time and the leadership of Abraham Lincoln and his generals Grant and Sheridan. We know how the South was defeated, slavery ended, and the Union restored.
During this same period in Mexico, France had invaded and imposed Maximillian as emperor. Despite the short-lived Cinco de Mayo victory in 1862, President Benito Juarez was eventually forced to retreat to the border where, with a ragtag army of guerrillas, he sought to reclaim the Republic for the next five years. Most students of Mexican history know that story.
What neither group knows is that our two countries came together during this period, not by government edict, but by volunteers who saw the destruction of either republic as a threat to their freedom. And this was a time when both countries had good reasons not to trust each other and, while past resentments and prejudice on both sides existed, so did common sense and self-interest.
While Lincoln certainly sympathized with Juarez and the Mexican struggle, he was too preoccupied with preserving the Union and keeping France from joining up with the Confederacy to take any overt action. He did, however, encourage clandestine movements such as fundraising by pro-Mexico groups, and even delivery of arms and munitions to Juarez.