The 11th, 12th & 13th Presidents of the United States had the most to do with California going from Mexico to the United States.
James Polk, 1845-1849, was extremely religious, believed in Manifest Destiny, and believed everything in North America should belong to the United States. He was a "Jacksonian" - no federan banks. Because of his desire to expand the United States we nearly went to war with Great Britan over Oregon - "54/40 or Fight." In 1845 he sent Freemont to look at California where upon the Mixicans forced Freemont out of California and in 1846 Polk ordered him back to California. The Brazos River used to be thought of as the Texas boundary but Polk pushed the Texas boundary to the Rio Grand.
Although Zackary Taylor did little to expand California, he met Freemont to figure out where Indian lands were and made recommendations for Freemont to be sent other places; part of that decision was because Freemont was a good cartographer (map maker). Taylor would not sign the Clay Compromise of 1850 and said he would veto it, even though it contained granting California statehood. Taylor worked beproj with engineers and England in 1850 to build a canal in the ithsmus of Panama but it could not pass the house and the bill died in 1881. Taylor was a penny pincher and wore clothes with patches. P.T. Barnum once offered him $10 for his pants; although the family had over 6000 acres in Kentucky, he was still tight, and, had slaves, so, obviously he was not against slavery, but, he did not like the idea of soldiers in the north returning slaves to the south.
Fillmore, a Wig,a.k.a., the do nothing party, a.k.a., the know nothing party, which became the American Party, which became the Republican Party, had two and a half years after Taylor died in office and signed the 1850 compromise that Taylor wouldn't sign which said could recover slaves but must have an I.D. saying who they are and coull hold in municipal jails. He wanted to run for a second term but was not nominated. He was railroad friendly - sigined the act that allowed the railroad to hold ¼ mile on each side of the center line of the tracks. Often misaligned, a very good business man and knew how to work congress; had more cabinet members than any other president; if member did not perform - canned and replaced.
So, these three presidents are the ones who had the most to do withCalifornia becoming a state.
Brother Don "Peanut Butter" Breeland, well known Historian Extraordinaire