White southerners were not just content to extend slavery across the continental United States. No they wanted more and they looked South to the Caribbean and Latin America to grow their "Empire of Slavery." In the 1850s, white southerners stepped up their aggressive expansionism.Filibusters, led by men like William Walker, raised private mercenary armies in an attempt to seize countries in South America. William Walker declared a short lived republic in the Mexican states of Sonora and Baja California in 1853. In1855, he was hired by a faction in the Nicaraguan Civil War after the country was being considered for the site of a canal to link the Caribbean with the Pacific Ocean. In 1856, Walker declared himself the commander of the country’s army and later the President of the Nicaraguan Republic. After attempting to take control of the rest of Central America, an alliance of four nations defeated him, and he was later executed by Honduran authorities in1860. Filibusters like Walker exemplified the power of martial masculinity and white supremacy of southern slaveowners that embarked on aggressive expansionists foreign policies designed to expand slavery into Latin America. 

Furthermore, white southerners dreamed of annexing Cuba. In fact, as far back as Thomas Jefferson, many Americans had wanted to seize the island, due to its lucrative sugar plantations. The Young America Movement of the 1830s and 40s and southern politicians like John C. Calhoun, argued that Cuba should be annexed, not only to protect the United States, but also to expand slavery therein. Filibusters to the island failed from 1849 to 1851. The strongest support for the annexation of Cuba occurred during Franklin Pierce's administration. Pierce was a northern Democrat from New Hampshire, but he was a pro-southern president who turned a blind eye to American filibusters and actively supported the acquisition of the island. Pierce and others viewed this as necessary, since northerners and southerners were arguing over the passage of the Kansas-Nebraska Act, which effectively destroyed the Missouri Compromise and once again increased agitation over the extension of slavery into the territories of the Louisiana Purchase. Cuba seemed like a way to placate white southerners, so members of Pierce's administration drafted the Ostend Manifesto. This called for either the purchase of Cuba from Spain, or the conquest of the island. The document was later published, causing wide spread outrage in Europe and the North. This irreparably damaged the Pierce administration and was one major factor in the creation of a new anti-slavery party, the Republican party. 

As the political crisis of the 1850s deepened, America veered towards Civil War. While it was not a foregone conclusion that war would occur, numerous factors, such as Bleeding Kansas, the Sumner Brooks Affair, the Dred Scott decision, and the Raid of Harper's Ferry by John Brown, all contributed to the increased animosity between the sections. To many white southerners, the election of Republican candidate Abraham Lincoln, who did not appear on ballots in the South, seemed to be the last straw. It did not matter that Lincoln had pledged no to touch slavery where it already existed, merely to stop its extension into the West. Perception often trumps reality and white southerners believed his election would hurt their peculiar institution. Thus, South Carolina seceded in December 1861, followed by other states in the Deep South. 

Some politicians outside of Lincoln's administration attempted to compromise. John C. Crittenden from KY proposed his Crittenden Compromise, which would have created an unamendable amendment to the constitution. This would have recreated the 36'30 line, but also allowed the extension of slavery South of it into any territory yet to be acquired. The meaning was clear, this would allow future annexed regions, such as Cuba, to become U.S. territories and later slave states. Lincoln rejected this and continued to pledge his non-interference with slavery where it existed. But white southerners in the Deep South did not believe this and were determined to protect their sinful institution. We can see this in every single secession convention declaration, as well as the "Cornerstone" Speech by Confederate Vice President Alexander Stephens. These powerful elites agreed that slavery was the "cause of the late rupture" and that their rebellion was specifically meant to create a new government to protect slavery.

After the firing on Fort Sumter, "Civil War was inaugurated at last." While most people focus on the war in the East, out in the Far West, the Confederate government immediately took an interest in extending their empire of slavery. John Baylor of Texas invaded New Mexico territory and declared the Confederate territory of Arizona, which would protect slavery there. Back in Richmond, Henry Sibley convinced Confederate President Jefferson Davis to allow an expedition to the American Southwest in order to bring the region under Confederate control. The Confederate government not only wanted to extend their empire of slavery here, but also wanted the deep water ports of CA in order to transport cotton, since these ports were not yet blockaded by the U.S. Navy. In addition, the Confederates wanted the regions vast mineral wealth in order to fund their war. In the end, the Confederate invasions were turned back and the Confederacy's dream of an Empire of Slavery in the American Southwest died. Regardless, throughout the war, white southerners wrote numerous policy papers, newspaper articles, and other statements about their desire to extend slavery into the Caribbean and Latin America, should they be victorious. While these were just pipe dreams, it was still a prominent conversation among white southern elites and journalists, and signals to us that these men would have extended slavery into those regions, had they not been defeated. 

The point, is that throughout the antebellum era and even into the war itself, white southerners dreamed of an Empire of Slavery. The political conflicts of the antebellum era revolved around slavery's extension into the territories. When these were resisted, filibusters and threats were used in an attempt to take over Latin American and Caribbean countries to extend slavery there. Even in the Civil War, Confederates widely discussed expanding their Empire of Slavery into the American Southwest and beyond. Had it not been for the war, these men would have been successful and instead of destroying the sin of slavery in 1865, it very likely would still exist in some form to this day.

White southerners had dreamed of establishing an "Empire of Slavery" for decades preceding the American Civil War. Manifest Destiny was a powerful concept in the United States, dating back to the early 1800s. Most Americans believed it was the destiny of the country to possess and control the entire continent and the government took steps to acquire this territory, as seen in the Louisiana Purchase and other policies. Northerners obviously wanted to "open up the west" for free-soil small farmers and entrepreneurs. But many white southerners wanted to extend slavery in areas deemed hospitable to plantation agriculture in the continental United States.

With Indian removal, which entailed ethnic cleansing and genocide in the process of nation-state building, new territories were opened to white settlement. This intensified sectionalism from the 1820s onward, as Americans debated the extension of slavery into the territories of the Louisiana Purchase. White southerners looked for ways to maintain their political power, while also perpetuating their peculiar institution. White southerners knew they were being outnumbered in the House of Representatives, due to the ever increasing numbers of white northerners and westerners, which was also bolstered by European immigrants to those regions. As western states came into the Union as either slave or free, these white southerners wanted to keep a balance of power in the Senate. This precipitated such episodes as the Missouri Compromise, which determined where future slave states could be formed. As a result new slave states were formed, such as MO, AR, AL, LS, and MS.

But this was not enough, Many white Americans wanted to annex the new slaveholding Texas Republic, which had won its independence from the Republic of Mexico. After nearly a decade of debate, the United States annexed Texas, which caused a war with Mexico. In the aftermath of American victory, the United States obtained the "Mexican Cession," which contained modern day CA, NM, AZ, CO, west TX, NV, and more. Since the Missouri Compromise only pertained to the Louisiana Purchase, a new agreement needed to be made pertaining to slavery's extension into the Mexican Cession. During the war, David Wilmot of PA attempted to place a "proviso" on war funding, which would have forbade slavery's extension into any territory gained from the Mexican American War. This infuriated white southerners, who wanted to extend slavery into this new profitable territory. A compromise was reached in 1850 that stated popular sovereignty, the process where settlers of various western territories would vote on whether or not their state would be a slave state or free, would be the litmus test for slavery's extension into the cession. This satisfied no one, as the compromise had other provisions that were onerous to both white northerners and southerners. Thus, the next decade would see several political battles over slavery's extension.

Sorry my answer was too long so I had to break it up. Start with the middle piece, then the top piece, and the sources are at the bottom.

For more information, see:

Brown, Charles Henry. Agents of Manifest Destiny: The Lives and Times of the Filibusters. Chapel Hill: University of North Carolina Press,1980.

Freehling, William W. The Road to Disunion II: Secessionists Triumphant. New York: Oxford University Press, 1990.

May, Robert E. The Southern Dream of a Caribbean Empire, 1854-1861.Baton Rouge: Louisiana State University Press, 1973.

Nelson, Megan Kate. The Three-Cornered War. New York: Scribner, 2020.

Potter, David M. The Impending Crisis, 1848-1861. New York:Harper and Row, 1976.