When the Mexican-American War began in 1846, several hundred recently immigrated Irish, German, and other Roman Catholic Americans who were sent by the U.S. government to fight against Mexico came to question why they were fighting against a Catholic country for a Protestant one, combined with resentment over their treatment by their Anglo-Protestant officers, and deserted to join forces with Mexico. Led by Captain Jon Riley of County Galway, they called themselves St. Patrick's Battalion (in Spanish, Batallón de San Patricio)[25] and frequently sang the song "Green Grow the Rushes, O".
One theory has it that there was no mention of the word "gringo", in any publication in New Spain, or what would later be Mexico, until 1847, following the US occupation of northern Mexico during the Mexican–American War.[citation needed] Marching songs reportedly sung by US soldiers, such as "(Green Grows the Grass in) My Old Kentucky Home" and "Green Grow the Lilacs", became popular in Mexico.[26] This theory also has it that native Spanish speakers have difficulty pronouncing a second "r" in "green grows", which was elided as "green-gos". Shortly afterwards Mexico City newspapers like El Universal and Excelsior began to use the word "gringos" for Americans.
The 3rd Cavalry were the only U.S. Cavalry unit to wear green stripes on their trousers, and some believed that during their campaigns in the Southwest they were referred to as Gringos because of that stripe. Because of the prominence of Irish Americans in the regiment, the regimental song was "Green Goes the Rushes, Ho"
Gringo Wikiedit:
I served in the 3rd Cav and remembered reading something about this in our regimental history booklet. I believe that little booklet gave all the credit to 3rd Cav troopers for this.