Some Americans thought that making California a part of the US would strengthen the security of the nation because there was going to be fewer war threats by adding more territory.
Step-by-step explanation
They believed that it would protect them more from war threats because California provided a coastline to protect the country, which added more territory. Having the control of it, they could build a defense against other countries who would want to invade from the west side.
In addition, California's natural resources such as silver, gold, and fertile soil were very important for the nation, which was in a growing process.