scholar
Banned
That's not what happened, actually. The causes of an American Civil War existed before the civil war and only became more pronounced after the inherent failure of the Missouri Compromise to keep the North and the South in balance. California's refusal to become a slave state, or give would be slave states some of its territory was a major problem. Another was that after the Missouri Compromise was done away with, neither side played fairly in allowing the territories to choose for themselves to be slave states or not to be slave states. Various sympathetic acts of terrorism against the South also comes to mind. And then there was the apparent battle lines being drawn by the parties, and a lack of motivation to compromise with the South, which was increasingly getting weaker. In the end they seceded because their power over the country had been marginalized and they believed, rightly or wrongly, that the north would end their slave rights on a sanctimonious moral crusade that they believed, wrongly, had no business in politics and was a flagrant violation of States Rights.It's worth noting that IOTL merely annexing half of Mexico touched off the Civil War. ITTL the USA would start disintegrating from the expense and culture-crisis caused by raising an army large enough to sit on Mexico forever.
Granting the South more mostly empty land, land that won't object to becoming Slave States, alongside making California a Slave State (delay the discovery of gold, more promotion of Southern Emigration), and the Civil War could reasonably be avoided for decades. The Failure of the Missouri Compromise would still become apparent inside of Mexico, but enough of a bone to the South could be given to keep up the mutual feeling of compromise, and there is also the very likely prospect that while the North and the Deep South agree on the stance of the abolition of slavery that they will not agree on much else, adding a third dynamic to internal politics rather than North and South [as the West, while it captured the minds of pulp readers everywhere, did not hold much political power even in California until well after the civil war].