Apropos of my recent (I thought well-deserved) trashing of Millard Fillmore, my grad-school friend Chris Paine argued that had Zachary Taylor lived, and Fillmore not succeeded him in 1850, the Civil War would have started 10 years early, and the North would have lost. I was prepared to concede the first point, which Mark Stegmeier made pretty persuasively (in Texas, New Mexico, and the Compromise of 1850 [Kent State U. Press, 1996]). Prof. Stegmeier believes the flashpoint of an early American Civil War would have been Santa Fe, capital of the former Mexican province of New Mexico, whose post-Mexican-War ownership was disputed by the state of Texas and the U.S. government. In 1850, as Southern whites threatened secession over California statehood, and Congress deadlocked on a package of compromise bills, President Taylor sent 600 federal soldiers to Santa Fe. Governor Bell of Texas organized his own military expedition, and Southern politicians and newspapers endorsed Texas's claim and threatened war in the event of a clash between Texan and U.S. forces. The final Compromise of 1850, which Fillmore signed, included a peaceful resolution of Texas's boundary dispute with New Mexico, which, along with some horse-trading about slavery in the Mexican Cession, quieted Southern hot-heads and prevented a war. Taylor, however, opposed much of the Compromise and would have rejected it if he had lived; he probably also would have started a war with Texas, which several of the more bellicose Southern states would surely have joined.
I was less inclined to believe that the South would have won an early conflict with the Union, because, however underdeveloped the North's factories and railroads were in 1850 compared with their 1860 counterparts, the determinative factor in the war of 1861-65 was not industry but manpower, and in 1850 the free states already had twice as many people as the slave states. Fergus Bordewich's book America's Great Debate (Simon & Schuster, 2012), however, disabused me of the idea that the North's demographic advantage would have been enough to win a war with the South. What would have mattered most of all in such a war, Bordewich argues, was the willingness of large numbers of free-state or slave-state men to fight, and Northerners simply weren't very interested in fighting for the Union in 1850 (pp. 393-394). In 1861, by contrast, thousands of free-state men were eager to stomp Southern whites' guts out, thanks to a decade of "Slave Power" outrages: the vicious over-enforcement of the Fugitive Slave Law, the Kansas-Nebraska Act, the Lecompton Constitution, the Dred Scott decision, and the caning of Senator Charles Sumner. In 1850, martial keenness was all on the southern side of the Mason-Dixon Line: Southern whites had contributed tens of thousands of troops to the Mexican War, and many were threatening violence if California entered the Union as a free state or Texas didn't get to annex most of New Mexico. The North had more people, but not necessarily more willing soldiers, than the South.
If war had broken out between the states in 1850, it would have started in Texas and spread eastward as some of the slave states announced their secession and seized federal forts and arsenals.
I suspect the seceding states would be pretty much the same crew that first left the Union in 1860-61: the Deep South states from Texas to Florida, plus Georgia and South Carolina, and possibly including Tennessee (which had strong ties to Texas) and Arkansas. I also suspect there would have been some resistance by federal soldiers to the seizure of their posts, as President Taylor, unlike President Buchanan ten years later, would probably have ordered his men to defend their positions. It's hard to determine how the rest of the war would have unfolded, but I think that one could draw an analogy between the Civil War of 1850 and the War of 1812: reluctance on the part of Union men to enlist, several states (chiefly the slave states that remained in the Union) disaffected to the point of mutiny, increasing difficulty passing war-finance measures in the Union Congress, and, on the tactical side of things, a lot of small battles around the borders of the region the U.S. was trying to conquer, but little territory changing hands.
I can't see the secessionists gaining any of the advantages in the 1850s that might have delivered a knock-out blow to the Union in the 1860s war: European recognition or capture of the Union capital. The British government (which had a Liberal plurality in the early '50s) was only mildly interventionist, the French were under the unstable Second Republic until late 1851, most other European governments were uninterested in American affairs, and the Confederates would have found it difficult to take Washington if, as I suspect would have been the case, Virginia had nominally remained in the Union (as a de facto "neutral").
One particular political event, though, would have helped bring an early end to the conflict, and that was the presidential election of 1852. Absent an early Union victory it is unlikely that a Whig candidate for president could have won election in the U.S., even if (especially if) Whig incumbent Zachary Taylor decided to stand for another term. The new Democratic president would owe much of his political support to Democratic grandees in those slave states still in the Union, few of whom would have wanted the war to drag on, and would be under great pressure to negotiate with the rebels. I suspect the new president would have arranged an armistice with the secessionists in 1853 and a peace treaty in 1853 or '54, granting them their independence (perhaps with an empty clause allowing the seceded states to re-enter the Union in future) with provisions for the repayment of Southerners' debts and for the return of runaway slaves (both of which would probably have been dead letters, as with their counterparts in the Anglo-American treaty of 1783).
What of the war's aftermath? Here I think Bordewich is right to paint a bleak picture of American prospects. The United States would not go on to become a world power by the early twentieth century, if ever. Even given favorable trade terms with the new Southern confederacy, the Union would not have the same access to cheap Southern raw materials (timber, iron, coal, cotton) that helped fuel the industrial expansion of 1865-1900. The confederacy probably would have been able to conquer and hold New Mexico and make it difficult for the Union to send settlers, administrators, or troops to Oregon and California during the war; I suspect Californians and Oregonians would have considered seceding from the Union themselves, and there's a small chance that Britain would seize California during or after the conflict, using "the protection of British interests" as the pretext. (California had several hundred million dollars in gold to serve as the real motive.)
As for the confederacy itself, life would remain much the same there after the war as before for a very large percentage of the population – namely, African-American slaves. Some slaves would have been able to escape during the secession war (as during the Revolution and the War of 1812), but it's unlikely that, given the limited opposition to slavery in the North in 1850, the Union Congress would have passed Confiscation Acts mandating that generals free runaways, or that many Union generals would have done so on their own. I can, however, see free states in the Union refusing to return runaway slaves to rebels or allow slave-catchers into their territory after the war, and abolitionists maintaining an Underground Railroad into the free states of the Union.
Beyond the borders of the old United States, a secessionist victory would certainly have had significant consequences. The 1850s historically saw a number of American filibustering expeditions against Central America and Spanish Cuba, and while few of these would have occurred during the secession war, there is little reason to assume that citizens of the newly-independent Southern confederacy would not have attempted to expand their nation southward by military conquest. The confederate government might even have quietly backed some of these expeditions, since expanding the domain of American slavery was important to many white Southern leaders in the 1850s; with state backing, private efforts to grab Nicaragua or part of Mexico or might have succeeded. Meanwhile, as the war would have disrupted the domestic American slave trade - the U.S. government would not have allowed those slave states which stayed in the Union to export slaves - the confederates would probably also have tried to reopen the African slave trade, either directly or using Brazil (which continued to import African slaves until ca. 1870) as an intermediary. The Southern confederates' support in Britain, if any, would have shrunk as a result. Speaking of Britain, it is less likely that the British government would have endorsed dominion status for Canada, at least not as early as 1867, if it had not been worried about the possibility of a powerful United States conquering the Canadian colonies or luring them into the American Union. And it seems equally unlikely that, if Napoleon III had decided (as he did in our continuum) to put Maximilian on the Mexican throne in 1861, he would have been sufficiently intimidated by the weakened United States - or the confederacy - to withdraw his support for a Mexican empire. The Monroe Doctrine might well have been a dead letter in the aftermath of our early Civil War.
On the positive side, there would not have been any re-enactors of this Civil War. Thank heaven for small favors.