A French unit did join the Confederates in the Battle of Palmito Ranch in 1865—the only time that an organized military force from a foreign country intervened in the war. This Confederate victory was the last battle of the war.
Thanks for that info. I was just talking with my kids about World War I, and quipped that I wonder if the Revolutionary War might have been close to being considered a “world war”.
Then they asked about the Civil War, and I told them I didn’t think there was much involvement by other countries.
I don't know that I'd call that an "intervention." The story seems to be this: An idiotic desk jockey of an officer, newly assigned to the US unit and desperate for some glory before the war ended, violated an informal truce as everyone waited for news from the east that it was all over (Lee had surrendered a month earlier). His "sneaky" move along the Rio Grande was spotted by French border troops, who tipped off the confederates. The US forces--who could see the French watching them across the river and should have known that the surprise was blown, walked into a trap, and in trying to escape, some tried to swim the river. The French troops fired, probably over their heads, to keep them back. It's unlikely that they hit any of them, since only four men were killed and dozen wounded in the whole battle.
After the war the US commander then tried to blame his subordinate, bringing him up for a court-martial. The subordinate called the confederate commander as witness, who blew apart the prosecution and put the blame squarely on the commander, revealing that he'd fled the battlefield.