I'm taking AP US History right now, and a few weeks ago as we talked about Polk's presidency the events leading up to the Civil War, I suggested jokingly that the Gadsden Purchase was actually the cause leading up to the south's secession.
Recently, I stumbled across more information about the motives behind the Gadsden Purchase. It turns out the land was purchased in order to make a southern route more feasible for the transcontinental railroad. This intensified the debate over whether or not the northern or southern route would be taken for the railroad, and led to higher sensationalist tensions in congress which led to the civil war.
There's a chance I may be entering a full-on debate with him about this topic, so I'd love some more input as to how the Gadsden purchase could have played a role in the path to secession.
The Gadsen Purchase was a cause of North South contention in the run-up to the civil war, but it was more of a minor quarrel, and not as important as the Fugitive Slave Act or the Kansas Nebraska Act in exacerbating the debates over slavery.
By the late 1840s, the Slave states were becoming worried about the growing probability that the parity in numbers (assuring equal votes in the Senate) between slave states and free states was going to break down.
In 1850 when California was admitted as a free state, the balance did break down. There were now 16 free states and 15 slave states.
Admission of California was accompanied by "The Compromise of 1850". This organized New Mexico (including most of what is now Arizona), and Utah (including what is now Nevada) as territories, with slavery to be decided in the future by popular sovereignty.
This left the slavery interests with only 3 areas for possible future slave states; New Mexico, Utah, and Indian Territory (Oklahoma). On the other hand, all the area that later became Minnesota, North Dakota, South Dakota, Kansas, Nebraska, Colorado, Wyoming, Montana, Idaho, Oregon and Washington, looked likely to eventually become free states.
Southerners were interested in gaining more territory that could become slave states. There were attempts made to gain land in Nicaragua, and to acquire Cuba, but these did not pan out. Gadsden was set to Mexico (By Jefferson Davis, who was Secretary of War at the time) to try to buy land from Mexico.
One of the reasons for trying to buy land was to gain a better route (avoiding mountains) for a southern Transcontinental railroad. This railroad would help the economy of the slave states (and was thought to help link New Mexico to the rest of the slavery empire).
Gadsden had instructions, however to buy as much land as possible (to gain territory for future expansion of slavery). Gadsden did not get Mexican president Santa Anna to agree to sell as much land as Gadsden was authorized to purchase, but he did negotiate a deal for 38,000 square miles of Mexico for $15m.
When Gadsden brought this deal back to Washington, it failed to get the required two thirds vote in the Senate to approve it. Northern senators thought that Gadsden had bought more land than was needed for the railroad, and suspected that he was really just trying to increase slave territory.
The US senate was already roiled over debate about the proposed Kansas Nebraska act (which was much more contentious), and enough northern senators voted against the Gadsden purchase to put a shot across the bows of the slave states, that it failed to pass.
Secretary Davis then persuaded President Pierce to ask the senate to pre-approve a modified deal of $10m for only 29,000 square miles of Mexico. This was voted through. Gadsden then returned to Mexico and agreed this new deal with Santa Anna.
That was how the US acquired the Gadsden purchase (which eventually made up about one third of the state of Arizona and a small part of New Mexico).
Northern resistance to the larger deal, which the Southerners hoped would add to slave territory, was another (of many) evidences to the South that Northern resistance to the idea of slavery and to the expansion of slavery was growing. It added to their list of frustrations and fears in trying to protect their slave empire. It was, however, a minor contribution to the causes of the Civil War, rather than a major contribution.
Sources:
http://en.wikipedia.org/wiki/File:US_Slave_Free_1789-1861.gif
http://en.wikipedia.org/wiki/Gadsden_Purchase
http://history.state.gov/milestones/1830-1860/gadsden-purchase
