Well, kind of. It came into the Union following the Compromise of 1850. This compromise was reached partially to deal with the territory acquired during the Mexican-American War which ended two years prior, and partially to deal with tension between Slave and Non-Slave states. Considering that the California and the other territory was already won following the Mexican-American War in 1848, California's admittance technically did not have any impact on westward expansion. However, the events the precipitated California's entrance into the Union were intertwined with westward expansion and the United State's victory in the Mexican-American War. So, in short, California's admittance was kind of the end result of the Mexican-American War, which was a big part of our westward expansion. Does this help answer your question? I am not sure if I articulated it very well, and it is kind of a complex situation. Let me know!