Battle of Monterey; Early on 7 July 1846, the frigate USS Savannah and the two sloops, USS Cyane and USS Levant of the United States Navy, captured Monterey and raised the flag of the United States. The only shots fired were a 21-gun salute to the new U.S. flag fired by each of the U.S. Navy ships in the harbor. Wikipedia

Battle of Monterrey; General Pedro de Ampudia and the Mexican Army of the North was defeated by the Army of Occupation, a force of United States Regulars, Volunteers and Texas Rangers under the command of General Zachary Taylor. The hard-fought urban combat led to heavy casualties on both sides. Wikipedia (General Taylor won the day!)

The Maid of Monterey - Published F. D. Benteen, Baltimore, 1851
She cast a look of anguish, On dying and on dead;
Her lap she made a pillow For those who groan'd and bled.
And when the dying soldier For one bright gleam did pray,
He bless'd the Senoretta, The maid of Monterey.

So, unless the song is completely fictitious, it appears to refer to the Battle of Monterrey.