I think you have the wrong war....the Spanish American War was a war fought 1898-1899 against Spain by the U.S. ...specifically it's Colonies...Cuba, Puerto Rico, Philippines is where the fighting took place. No fighting was "around the Mission areas of CA" (California). Read the history, the Mexicans defeated the Spanish in 1821.
The fighting in CA occurred during "The Mexican War" 1846-1848. But.... http://www.sscnet.ucla.edu/comm/steen/cogweb/Chumash/McWilliams.htmlhttp://www.sscnet.ucla.edu/comm/steen/cogweb/Chumash/McWilliams.html "Like most social changes in California, the transition from Mission to rancho was effected with uncommon rapidity and remarkable thoroughness. By 1848 the Mission system had been completely liquidated. Later the Supreme Court ruled that the Mexican governors and the administrators appointed to take charge of the Mission properties lacked authority to dispose of the church buildings, the homes of the priests, and lands to an extent of a few acres surrounding each of the twenty-one Missions. Eventually this moiety of the Mission estate was returned to the Archbishop of California; otherwise nothing remained of the Mission system,..."
So there were no "Mission areas" to be fought around in 1898-1899, the time of the SpAm War.
Search Mexican War at www.archive.org for old texts/books that have been scanned and put online.
Also if Castro was in the Mexican Army then look for Mexican Army History sites.