Many people are ignorant of history. They don't realize that there were many anglos living in California among the natives and Spanish (and Russians), and most didn't give a hoot about Mexico. Mexico had control for a while, if you can call it control (California was largely ignored by the Mexicans). The Spanish here in California worked with the anglos to declare independence from Mexico. General Mariano Vallejo played a large part by wanting California to become part of the USA, and became part of the new government, built a new capital in Vallejo named after him. Californians gladly became part of the USA. Also, California was not taken away, it was paid for after the war by treaty, where the USA paid almost $20 million to Mexico for territories lost. Which was a pretty good deal for Mexico, having lost the war they caused.
Vallejo is an unknown hero. He was a real European liberal, what we now call "conservative".
Don't know that he gave up 250,000 acres in Sonoma all that willingly but he definitely wanted to be free and work with the Americans.
Treated badly at first but eventually regained his respect from the Anglo Californians.
And the US moving forces into the disputed Nueces Strip had nothing to do with it. Interestingly, the US had, 6 months earlier, tried to buy the Strip from Mexico, which would seem to be a concession that they didn't already own it.