Katt Williams was wrong

Mexicans did fight for California. In fact, the one major battle they had with Anglo forces invading California they won, with horses and lances, just outside of Los Angeles. Unfortunately, the decision to turn the state over to the United States was made in Washington D.C. without the input of the people involved.
In fact, there was a whole war that Mexicans fought to stop the illegal invasion, which, lest Mr. Williams forget, was being pushed by the slave-owning interests in the United States. It was Southern slaveholders who ignited the war to rip Texas away from Mexico when Anglos refused to accept Mexico's laws against slavery.
Mexico had abolished slavery in the early 1800s, way before the Emancipation Proclamation; Mexico even had at least two African-Mexicans as presidents some two hundreds years before Barack Obama was elected president in this country.
The main catalyst for the Mexican war was the refusal of Mexico to return black slaves--believed to be more than 10,000--who had taken the southern-route of the "underground railroad," crossing the border to a free Mexico. In Mexico's governing assembly heavy debates on the issue ended up with the majority supporting these slaves, allowing them to own land, to farm, to become part of the Mexican social fabric.
Mexicans were willing to die so blacks could be free.