I was recently reading "Duty, Honor and Country: The Civil War Experiences of Captain William P. Black, Thirty-Seventh Illinois Infantry" [positive review forthcoming in Civil War News] and he relates an interesting episode I'd never heard of before.
In Black's letter of January 14, 1864 he talks about the January 12th battle of Matamoras, Mexico. At the time Black was stationed in Brownsville, Texas and it appears that the Union soldiers traveled back and forth from Brownsville to Matamoras all the time. The local Mexican leaders were in the midst of a power struggle which came to a head on January 12th.
Juan Cortina led a force of 600 men and 6 cannon against the current Governor Manuel Ruiz's 800 men and 4 cannon. The battle lasted about 18 hours but they took an hour break when Union soldiers arrived on the scene. The US Consul asked for a guard from the Union commander in Brownsville and he sent 4 companies of the 20th Wisconsin, 94th Illinois and Battery B 1st Missouri Artillery. The consul was worried for his family's safety as well as the safety of the roughly 1 million dollars he had in gold in silver. When the Union soldiers arrived both Mexican forces stopped fighting and sent couriers to the Americans to see why they were there. When the commanding officer convinced the Mexicans that they were only there to protect the consul the Mexicans went back to their own battle. Later the Union soldiers transported the consul and the gold and silver to Brownsville.
Cortina won the battle and became the new governor. Ruiz fled to Brownsville. There were roughly 150-200 Mexican casualties.
I found this little battle amazing because it seems so weird. The Union soldiers rush into foreign territory to protect their consul and the specie (probably more so the specie). And then the two Mexican forces pause their battle to find out what this third force intends to do. I'd never heard of it before, just goes to show you can learn something new everyday.