The South was enthusiastic for a war with Mexico prior to its outbreak in 1848. There was bone deep bitterness between The Southern slaveocracy and the Mexican government. It heartened back to the Mexican Revolution of 1820. Two irritants to Southerners emanated from the Revolution.
First, the fact that white aristocrats had been defeated and deposed by non-white people’s who were forming a government of their own. This was the second time this happened in the Americas in the 1800s. The first instance was in Haiti. Although it brought the USA the Louisiana Purchase, a black republic in Haiti struck fear of insidious genocide against the slaveocracy.
The second irritant was the new Mexican government in Mexico City abolishing slavery. Slaves in the northern border states and the Tidewater Region ran away to the free north, Pennsylvania and the states of the Northwest Territory. Slaves in the Deep South, Alabama, Mississippi, and Louisiana might likely head to Mexico.
Just as the runaway situation created severe tensions between Northern and Southern states, it created even more profound tensions between Mexican and Southern slaveocracy politicians. Both sides craved a fight. Mexican elites roundly disparaged Southerner of the slaveocracy as unmanly and cowards. In turn, the Southern slaveocracy despised the Mexicans as degenerate subhumans. They were also view as the major impediment to the Purple Dream.
The Mexican American War was a spectacular triumph for Southern Arms. Almost every important CSA General was an officer in that war. The only Union West Point officer was US Grant.
The South felt it was cheated of the benefits of the war by the Compromise of 1850, especially given the support for the compromise on the part of a Southern plantation owing military hero of the war, President Zachary Taylor. Securing the South against the North became an obsession in the South. The South was “loosing” the Southwest and thence the West itself.
Senator Stephen A Douglas, a rising star in the Senate, a political foe of Senator Atchison, the Democratic senator from Missouri and frequent president pro tem of the Senate, and a lawyer for the Illinois central railroad, was proposing a transcontinental railroad. He was obviously proposing a northern route which with cement a hold of the North upon the West. Southerner, Jefferson Davis, was among a group of Southern politicians who wanted to cut the North off from easy access while guaranteeing to the South. As Secretary of War, Jefferson Davis pushed Congress to buy a strip of land from Mexico after the settlement of the war, which was necessary for any such venture.
The purchase of that land in 1854, was the South’s attempt to link the East Coast’s expanding rail complex to the West Coast via the South. Jefferson Davis got the land deal. The South didn’t get the transcontinental railroad as Douglas’ influence saw to a northern route.
The failure to see through the Southern transcontinental railroad route as the official first route was a major insult to slaveocracy politicians and a sign that the South was increasingly out of synchronization with the rest of the Union. It was a major blow to the expansion of slavery and the idea of the Purple Dream.