They overwhelmed and captured the small Mexican garrison at Sonoma and declared the California Republic (Spanish: La República de California), or Bear Flag Republic, raising the original Bear State flag over the captured garrison.

On July 5, 1846, Brevet Captain John C. Frémont assumed control of the republic's forces and integrated them into his California Battalion.

[9][10] The issue of the expansion or restriction of slavery was a fundamental dispute in the admission of new states to the Union since before the passage of the Missouri Compromise in 1820.

General Zachary Taylor, a hero of but also a staunch opponent of the Mexican War, had become President in March 1849.

It is probable that the intercourse between those countries and our possessions in that quarter, particularly with the Republic of Chili, will become extensive and mutually advantageous in proportion as California and Oregon shall increase in population and wealth.

… No civil government having been provided by Congress for California, the people of that Territory, impelled by the necessities of their political condition, recently met in convention for the purpose of forming a constitution and State government, which the latest advices give me reason to suppose has been accomplished; and it is believed they will shortly apply for the admission of California into the Union as a sovereign State.

[15] Despite outlawing slavery and applying to the Union as a free state, California had elected one anti-slavery and one pro-slavery senator, John C. Frémont and William Gwin, respectively.

Although Fillmore was a northerner and was not a slaveholder, he had strong ties to the South and was much more open to a compromise that would allow the extension of slavery into the territories.

The change of leadership opened the door for the passage of the Compromise of 1850, crafted by Senator Henry Clay, which allowed the admission of California into the Union without Congress imposing any limitation on the introduction of slavery.

[18][19] On January 29, 1850, Senator Clay proposed eight resolutions to end the heated debate over the status of the territories acquired from Mexico.

[14] On September 11, 1850, Edward Gilbert (Democrat) and George W. Wright (Independent) took their oaths of office to become the state’s two Representatives.