What if Buchanan/Lincoln had let them go?

[Sea Skimmer]

The Confederates were well aware of the long term trading potential with Asia, which was already exploding in the two decades prior to the USCW, as well as simply not wishing to be hemmed in. Northern Mexico means little, a path to the sea and eventual transcontinental railroad had massive value. This is why after the Mexican war proper they were trying to get a law passed to make all unincorporated territory south of a certain parallel declared slave by default. I forget which it was, but it would have given them a small sliver clear across the country.

[Grandmaster Jogurt]

The Missouri Compromise forbade slavery above the 36°30' latitude and let it be legal south of it, but that was signed in 1820, and the Compromise of 1850 avoided the law being passed which would have banned former Mexican territory from being made into slave states. Are you thinking about one of these or is there an event that I missed?

[Sea Skimmer]

Figured it out, rejected portion of the Compromise of 1850, which would have among other things split the state of California along 35 degrees N and made the southern piece a slave territory.

[The Duchess of Zeon]

Fascinating bit of alternate history there. There might have been a "civil war in a pocket" (since the Confederate sympathizing Californians would be concentrated in the southern state and joined by a lot more) separated from the rest of it, with North California and Oregon regiments versus Confederate California. I doubt it would have changed anything in terms of Confederate victory prospects, or even the slid toward war, unless it actually accelerated it. Would see a very different US west coast today, though, with LA/San Diego in one state and San Francisco/Sacrament and the central valleys in another.

[Sea Skimmer]

I cant see the Confederates lasting long unless they have a major starting advantage in arms and ammunition, unless perhaps the Mormons also rose a full scale rebellion. Historically Union troops pull out of Utah in 1861, only to return a year later and build a fort overlooking the Mormon Church, so a window of vulnerability existed in which the Mormons could organize as they pleased, but a rebellion would mean nothing to the Confederates unless they actively went out and attacked Sacramento. I can't see them doing it, they just wanted to be left alone and semi independent.

Perhaps this would be more plausible though if Confederate agitators arranged stockpiling of armaments before the outbreak of war, but I'm not aware of anyone actually trying to do this historically. Probably because anyone so inclined realized in the east that they could never hope to make a useful scale of preparation compared to what was held in federal and state arsenals throughout the land. In the west it would be a bit different, though California still had 380,000 people in 1860, I just checked. So it would need to be I dunno, ten thousand rifles and several artillery batteries delivered. That and ammunition could fit on one or two ships.

[Guardsman Bass]

Sonora is a pretty good region for agriculture in northern Mexico, particularly if they made the investments to properly manage water (it produces about 40% of Mexico's grain). There are also valuable minerals in its mountains. Tamaulipas has Monterrey.

Sure, they're not getting a ton of immigrants at this point, but there are a lot of poorer southern whites who would likely take a stab at turning it into cotton land. The bigger problem is that the Confederacy can't really afford to make war on Mexico while it's under the occupied rule of Maximilian I (installed with French support), since they need European allies. He'll get toppled sooner or later after the French pull out their troops and Maximilian manages to alienate the Mexican conservatives who invited him and his people in, but that might take longer if the US is staring across the border at an independent Confederacy (the US backed Benito Juarez's Republicans).