From "The Birth of States" by Millard Donelson
Fremont Press, Albany, New York, 1939
"The state of Colorado is the first and, to date, only [1] state to have seceded from an existing state, in this case California, without that state having seceded first. The state of California, formed as part of the "Great Compromise" of 1850, was made a single state largely in order to placate the northerners who feared the extension of slavery to the part of the territory south of the Missouri Compromise line [2]; this fear was largely ungrounded, as the land there was completely unsuitable to plantation agriculture. The state remained divided in all but name; as such, the proposals to split the state were numerous throughout the 1850s.
In 1854, the State Assembly of California passed a bill to divide the state in three. The first state created would be an extended Colorado including Monterrey, Merced and Mariposa Counties and everything south of them. The second would be Shasta, consisting of the far north; this area, while quite populous at the time thanks to the Gold Rush of '49, was depopulated, and today has barely 300,000 inhabitants, and the third was the rump California in the middle. This bill failed to gain traction in the Senate, and was consequently abandoned; however, separatism remains in the far north of the state [3].
Andrés Pico, State Assemblyman from Los Angeles County.
It was five years later, however, that the movement gained traction. Andrés Pico, a rancher in the San Fernando Valley who had fought for the Mexicans against the US, was elected to the State Assembly in 1851, and campaigned for the division bill of 1854; although that bill failed, he continued to campaign for Coloradan separation. In 1859, he wrote a bill to split off the parts of the state south of the 6th Standard Parallel [4] as the Colorado Territory, since the area was not technically populous enough to be a state at the time [5]. This gained traction, and on April 18, Governor Stanford [6] signed the bill, it having passed both houses of the legislature. However, state secessions needed the support of Congress even before the Fifteenth Amendment [7], and thanks to the secession crisis, the bill proposed to Congress was ignored until September of the year after, when it was brought forth by Senator David C. Broderick [8]. It passed the House two weeks after, and the Senate by mid-October. Fremont signed it into law, formally creating the Territory of Colorado with Los Angeles as its capital, and Stephen C. Foster, long-time mayor of Los Angeles, was appointed as the first governor."
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[1] This is not to say that there won't be more before TTL's present day.
[2] ITTL, what with the state splitting so quickly, this view becomes common among historians, and indeed, it predominates both academic and popular history by the present day (ITTL, 'California' = the Central Valley and the Bay Area, and Colorado is seen as culturally separate, so it does make sense).
[3] The bill is OTL, and so is the rest of the paragraph; IOTL, the separatists advocate creating a new
State of Jefferson along with south-western Oregon.
[4] The Sixth Standard Parallel, used as the surveying baseline in California, is defined by Mount Diablo, and runs about 13.8 miles south of the 36th parallel north.
[5] The area had something like 30,000 people in it in 1860, and nearly half of them were in Los Angeles County, and something like a third in San Diego.
[6] IOTL, Leland Stanford lost the gubernatorial election of 1859 to John B. Weller, a Lecompton Democrat. ITTL, with the Republicans doing better generally in the state, Stanford wins the election.
[7] The Fifteenth Amendment, also known as the 'Perfect Union' amendment, made it significantly harder for states to secede. For more information, see Chapter XII.
[8] This is the first divergence (apart from Stanford being governor two years earlier); IOTL, the bill was never even discussed in Congress due to the secession crisis being later. As for Broderick, IOTL he got killed in a duel with David Terry, the former state chief justice, in 1859; ITTL, it's Terry who's killed, further damaging the Lecompton Democrat cause in California.