The impact of the Mexican War

The treaty of Guadalupe Hidalgo established the familiar territorial shape of the present-day US. The union gained a gigantic landmass from Mexico in the west, that makes up two-fifths of present territory. The only subsequent territorial gains in North America after the Mexican war were a small strip of additional land purchased from Mexico in 1853 known as the Gadsden purchase, and Alaska, which was purchased from Russia post-civil war and for long afterwards was not considered to be of any economic importance. The outcome of the Mexican War, however, was of great importance to the union.  Although the war had not been especially costly in military terms, it was the start of a train of events that, in the space of under 2 decades, led the nation to a civil war. The end of the Mexican war yet again divided northerners and southerners over the most important political issue of the time: slavery. inevitably, the key question was; should states created from this vast new territory become slave or free?

Prior to the Mexican cession, there had been no possibility of any extension of slavery. The last time that the union had been faced with this issue was after the Louisiana purchase of 1803; this had been resolved in the Missouri compromise of 1820 with the creation of the 36º30' line that divided northern and southern territories. In the late 1840's, due largely to high immigration rates from Europe in the US's northeastern cities, the population of the free states was increasing far more rapidly than in the southern slave states, thus, the slave states were becoming an ever-smaller minority in the house of representatives. In the senate, where states were equally represented regardless of population size (click here http://edexcela2historyrevisionusa.blogspot.co.uk/2016/04/slavery-and-us-constitution.html to read my post on slavery and the US constitution if you need more information on this issue!) the south retained much greater political power, but even here the equality between N/S found in the early years of the republic was threatened unless new slave states territory could be found. Additionally, the remaining territory from the Louisiana purchase that had yet to be organised into states - present day Kansas, Nebraska, Wyoming, and the Dakotas - was all north of the 36º30' line of latitude, and would therefore eventually have to enter the union as free states, tipping the delicate balance of free/slave states in the north's favour.  Therefore, the Mexican cession was a highly important issue for southerners. 

The Wilmot Proviso, 1846

In 1846, northern Democrat David Wilmot of Pennsylvania introduced a resolution to this issue that polarized sectional opinion. The 'Wilmot Proviso' stated that as slavery was outlawed in Mexico, slavery should be banned from any territory gained from Mexico. Wilmot, who was not an abolitionist, argued that he simply wanted to maintain the status quo. He was motivated, he explained, not by any "squeamish sensitivity upon the subject of slavery, nor morbid sympathy with the slave". Instead, he stated that his concern was that if slavery were legal in the new territories, the opportunities for free white labourers to settle and prosper there would be reduced. Additionally Wilmot insisted with complete plausibility that he was indifferent to slavery where it already existed. In any case, by the late 1840's, slavery was legal only in states, not territories, and Congress had no constitutional authority to interfere with slavery in a state. Territories were a different matter; no one up until now had disputed the right of Congress to settle the issue one way or another in areas that had not yet been granted full statehood. 

The proviso was a grenade lobbed into the delicate balance of the national two-party system. The potential for sectional break down was immediately apparent. Northern congressmen voted 83-12 in favour of the proviso, and southerners 67-2 against. The proviso passed the house of representatives on three occasions, but each time it was blocked in the senate, where the south still had the votes to wield a veto against it. Thus, as the proviso failed to pass the senate, it did not become law. Nevertheless, the proviso fuelled tensions, polarising opinions on both sides, with both northerners and southerners seeing their side of the question as not only a point of deep principle, but also an issue of great practical political significance. For anti-expansion and abolitionist forces, the proviso became a rallying cry. As for so many issues, opinion was sectional, with many northerners supporting the proviso and most southerners denouncing it.

The Calhoun Doctrine, February 1847:  

In the senate, John C. Calhoun - a campaigner of pro-states rights, who had been involved with the 1832 nullification crisis in South Carolina - presented a pro-southern view, that opposed the principles of the wilmot proviso: he argued that since the territories were the common property of all the states and were merely held in trust by Congress, southerners had a claim to the land and could not be discriminated against by being told that they could not take their particular species of 'property' (slaves) there with them. If the northern majority continued to violate the constitutional rights of the southern minority, he argued, then the southern states would have little option but to secede. 

The free soil party

One group of antislavery politicians saw an opportunity in this crisis over slavery expansion. For several years, antislavery reformers like Whiggish lawyer Charles Sumner had wanted to cut loose  from the system of compromise that existed within both of the two national parties and create a new "fusion" movement that would unite in one party those who opposed the extension of slavery. With what in retrospect appear to be unrealistically high hopes of breaking the old of the two-party system, delegates gathered in New York in August 1848 to found the free soil party. The nominee was former Democratic President Martin Van Buren, who brought with him faction of northern democrats who supported the Wilmot Proviso.

Popular Sovereignty 

If the national two-party system was to be preserved, and the sectional chasm that Wilmot had opened to be bridged, a new approach was needed, one that circumvented the Wilmot Proviso without endorsing Calhoun's doctrine, and that found a middle ground for northerners and southerners. Eventually, a solution was agreed to. The new states formed from new territories would decide on the issue of slavery themselves by popular vote. The phrase used to describe this policy - "popular sovereignty" - was coined by mid-western democratic senator Lewis Cass, and the idea was associated with him and senator Stephen Douglas. Consistent with democracy and self-government, this solution seemed to offer something for both northerners and southerners:

• It met the south's wish for federal non-intervention and held out the prospect that slavery might be extended to some of the Mexican territories. 
• It could be presented to northerners as an exclusion scheme; it seemed unlikely that settlers in the new territories would vote for the introduction of slavery. 

However, there were problems with the concept of popular sovereignty. Firstly, it went against previous practice. In the past, Congress had decided on what should happen in the territories. Did PS mean that Congress no longer held this power? There were practical difficulties too. The main issue was when exactly a territory should decide on the slavery question. Northern democrats envisioned the decision being made early, as soon as the first territorial assembly met. southern democrats, keen to ensure that slavery was allowed into the new territories, saw the decision as being made late, near the end of the territorial phase (after the territory had reached a population size of 60,000, when settlers were seeking admission into the union as states. In the interim, they envisioned that slavery would be recognised and protected. 
Despite this ambiguity, popular sovereignty was supported by many democrats. It was opposed by a few southerners who believed they had the constitutional right to take their 'property' anyplace they wanted, and by  some northerners who opposed slavery expansion under any circumstance.

The 1848 election

Much to everyones surprise, democratic nominee Cass was narrowly defeated in the 1848 election by the whig's nominee General Zachary Taylor, a Mexican war hero and Louisiana slaveholder. Essentially, the Whigs had decided that the only way to avoid further loss of support was to find an attractive candidate and to leave the squabbling about policy until after the election. The free soil party polled 10% of the popular vote, far more votes than the more radical Liberty Party had in 1844, but arguably had  less of an impact on the final result because they took votes in roughly equal numbers from Democrats and Whigs. The free soil party may have failed to win an election, but it was a movement of huge significance for the relationship between the slavery issue and the party system. What Free Soilers had realised was that the free states now had a sufficiently large population that if they could be united, they would produce sufficient Electoral College votes to elect a president even if the south voted as a block for someone else. For the first time in the short history of the American republic, a political party was making a serious bid for national power by appealing to only one section.