Was the Missouri Compromise too arbitrary to stop spreading of slavery to the west and the north?

The Missouri Compromise collapsed because it was "unfair" to the South, based on the "equities" discussed below.

Given the map below,

you can see that Missouri is on the same latitude as Kentucky, Virginia, Maryland, and Delaware, all slave states. (West Virginia, in the same latitude, later broke away from Virginia in order to become "free" because of its mountain altitude and resulting colder climate.) Logically, it would make sense that states in the same latitudes west of Missouri should have a chance as slavery as well. These include Kansas, Colorado, Utah, Nevada, and even California.

The reason that the South acquiesced in the arrangement was that except for Kansas, the future states west of Missouri were then part of Mexico (in 1820). The North had the advantage in that most of the states acquired in the Lousiana Purchase, (except for Lousiana, Missouri, and Arkansas and maybe Kansas) were likely to become free states. So the South wanted to get Missouri and Arkansas in "under the wire, as soon as possible, and ratified the Missouri Compromise in 1820.

Between 1820 and 1850, the North got three new free states, Michigan, Iowa, and Wisconsin, and the South got three new slave states, Arkansas, Florida, and Texas. What had been a balance of 12 free states and 12 slave states (in 1820) was 15-15, out to 1850. The admission of California as a free state in 1850 made it 16-15, and threatened the balance of power.

But the capture of a large stretch of land out to California in 1848 changed the whole picture. What what made things interesting was the lack of restriction against by the Compromise of 1850 in states immediately to the east of California on the same latitude as Kansas and Missouri (the Utah territory) plus modern New Mexico and Arizona (the New Mexico territory) Now the South had an incentive fight for Kansas (they didn't really care about Nebraska), to reestablish the possibility of slavery not only in that state, but modern Colorado, Utah, Nevada, Arizona, New Mexico, and Oklahoma.

First of all, the Missouri Compromise was not a law. It was a working agreement to avoid sectional strife over the extension of slavery into the territories. The pairing off of states meant that the balance in the Senate between slave and free states would not be upset for a time as well.

The Mexican cession added new land, and the question came up on how to handle the new land. California was a difficulty, but cutting the state in half and introducing slavery into a currently free zone was repugnant enough that the South didn't really press for it. Instead, the Compromise of 1850 let California in as free, in exchange for a Fugitive Slave law that the South wanted, but soon turned out to be something of a sour deal, since the enforcement made for provocative news that irritated both South and North. The Missouri compromise was then extended over the rest of the Mexican lands. This was something of a second sour deal for slavery enthusiasts, as few thought the barren lands of Arizona would suit the plantation lifestyle.

This 'peace' didn't last long. Stephen Douglas wanted to get the two territories of Kansas and Nebraska organized and did some horsetrading with politicians from Missouri. The prime slave land in the state was directly opposite the new Kansas land, and slaveowners were not keen on having free land a short distance away. Popular Sovereignty was the fig leaf to cover up this concession, and Douglas, who was tone-deaf to the extreme on the moral and political aspects here, went along. Far from being any type of moderate position, this was a direct violation of the 'deals' going back a generation and much of the North exploded in fury. Even most Southerners didn't see the point of a slave Kansas, seeing it as an untenable position.

To make matters worse, Missourians poured across the border not to settle, but to fill the ballot boxes of the "Popular Sovereignty" elections with thousands of fraudulent votes. One district with a few voters turned in a ballot with 1800 or so votes, all in the same pen and copied from the directory of Cincinnati Ohio. The free-soil settlers, soon far outnumbering the actual pro-slavery settlers - repudiated these votes and soon actual civil war erupted between the bulk of the settlers and the Missouri "Border Ruffians".

This conflict led to the creation of the Republican party, dedicated to the restriction of slavery into the current limits - a 'Free Soil' major party. The Democrats managed to tamp down the fighting before the elections of 1856 and won that election, but now the opposition had firmed up considerably and would likely win in 1860.

While the actual line was more or less arbitrary, it did more or less include all the territory that most people thought suited for slavery in the nation, and its extension really didn't add much more. Westward extension was geographically a dead issue due to the deserts, and expansion North or into other lands such as the Caribbean would provoke a fight to the finish with the Northern majority. The South needed to accept the situation that slavery would not be extended, or pick a fight. In 1857, with the Dred Scott decision and the Lecompton battle, they chose to fight, and broke up the Democratic Party.