Batallón de San Patricio was a unit of several hundred Irish and other soldiers of European descent, who deserted the US Army and fought as part of the Mexican army against the United States in the Mexican- American war of 1846 to 1848.
From the perspective of the Americans who fought the Mexican-American War, the "San Patricios" were considered traitors. But for the Mexicans of that generation, and generations to come, the San Patricios were heroes who came to the aid of fellow Catholics in need.
There are many arguments about why they switched sides, for example; free land offered by the Mexican government, harsh treatment by their superiors, not being allowed to attend mass, or possibly watching the conduct of their fellow soldiers after victories. With all the members dead now and none of us being there it's hard to say exactly why they deserted. And according to a t-shirt I saw this is why they did what they did "The St. Patrick's Battalion saw the Mexican-American war for what it was -- a land grab -- so they changed sides and fought on the side of the right and the weak. Even though they knew that they couldn't win, they still did what they felt was right." In letters written by John O'Riley who was a senior field officer for the battalion, he cites their common religion and a sympathy for the Mexican cause as it reminded them of political tensions and situations in their own native land.
He also describes the flag they fought under like this:
"In all my letter, I forgot to tell you under what banner we fought so bravely. It was that glorious emblem of native rights, that being the banner which should have floated over our native soil many years ago, it was St. Patrick, the Harp of Erin, the Shamrock upon a green field".
So regardless of how you celebrate St. Patrick's day, a day on which everyone is Irish(or wishes they were). Think fondly of these men who stood up for what they believed was right regardless of the consequences.
Erin Go Bragh(Ireland Forever)
