Title: Casus belli
Content:  While long-term conflict between the northern and southern states (mainly due to the economic disparities and moral questions caused by slavery) was the cause of the American Civil War, the Confederate attack on Fort Sumter (April 12–14, 1861) served as casus belli for igniting the deadliest war in American history.
Title: Origins of the American Civil War
Content:  primary cause of disunion, it was disunion itself that sparked the war." Pulitzer Prize-winning author David Potter wrote: "The problem for Americans who, in the age of Lincoln, wanted slaves to be free was not simply that southerners wanted the opposite, but that they themselves cherished a conflicting value: they wanted the Constitution, which protected slavery, to be honored, and the Union, which had fellowship with slaveholders, to be preserved. Thus they were committed to values that could not logically be reconciled." Other important factors were partisan politics, abolitionism, nullification versus secession, Southern and Northern nationalism, expansionism, economics, and modernization in the Antebellum period.
Title: American Civil War
Content:  The American Civil War (April 12, 1861 – May 9, 1865) (also known by other names) was a civil war in the United States fought between the Union (states that remained loyal to the federal union, or "the North") and the Confederacy (states that voted to secede, or "the South"). The central cause of the war was the status of slavery, especially the expansion of slavery into territories acquired as a result of the Louisiana Purchase and the Mexican–American War. On the eve of the civil war in 1860, four million of the 32 million Americans (~13%) were enslaved black 
Title: Origins of the American Civil War
Content:  all men everywhere could be free." On March 4, 1865, Lincoln said in his second inaugural address that slavery was the cause of the War: "One-eighth of the whole population were colored slaves, not distributed generally over the Union, but localized in the southern part of it. These slaves constituted a peculiar and powerful interest. All knew that this interest was somehow the cause of the war. To strengthen, perpetuate, and extend this interest was the object for which the insurgents would rend the Union even by war, while the Government claimed no right to do more than to restrict the territorial enlargement of it."
Title: Origins of the American Civil War
Content:  cornerstone rests, upon the great truth that the negro is not equal to the white man; that slavery—subordination to the superior race—is his natural and normal condition." In July 1863, as decisive campaigns were fought at Gettysburg and Vicksburg, Republican senator Charles Sumner re-dedicated his speech The Barbarism of Slavery and said that desire to preserve slavery was the sole cause of the war: "[T]here are two apparent rudiments to this war. One is Slavery and the other is State Rights. But the latter is only a cover for the former. If Slavery were out of the way there would be no trouble from State 
Title: American Civil War
Content:  Slavery was the main cause of disunion. Slavery had been a controversial issue during the framing of the Constitution but had been left unsettled. The issue of slavery had confounded the nation since its inception, and increasingly separated the United States into a slaveholding South and a free North. The issue was exacerbated by the rapid territorial expansion of the country, which repeatedly brought to the fore the issue of whether new territory should be slaveholding or free. The issue had dominated politics for decades leading up to the war. Key attempts to solve the issue included the Missouri Compromise and the Compromise of 1850, but these only postponed an inevitable showdown over slavery. The motivations of the average 
Title: Military history of the United States
Content:  Long-building tensions between the Northern and Southern States over slavery suddenly reached a climax after the 1860 election of Abraham Lincoln of the new anti-slavery Republican Party as U.S. President. Southern states seceded from the U.S. and formed a separate Confederacy. Within the Confederate states, many U.S. forts with garrisons still loyal to the Union were cut off. Fighting started in 1861 when Fort Sumter was fired upon. The American Civil War caught both sides unprepared. Neither the North's small standing army nor the South's scattered state militias were capable of winning a civil war. Both sides raced to raise armies—larger than any 
Title: Origins of the American Civil War
Content:  the North had aggressive and hostile designs on the Southern way of life. The South was thus increasingly fertile ground for secessionism. The Republicans' Whig-style personality-driven "hurrah" campaign helped stir hysteria in the slave states upon the emergence of Lincoln and intensify divisive tendencies, while Southern "fire eaters" gave credence to notions of the slave power conspiracy among Republican constituencies in the North and West. New Southern demands to re-open the African slave trade further fueled sectional tensions. From the early 1840s until the outbreak of the Civil War, the cost of slaves had been rising steadily. Meanwhile, the price of 
Title: Origins of the American Civil War
Content:  disagree sharply on the aspects of this conflict (ideological, economic, political, or social) that were most important. The principal political battle leading to Southern secession was over whether slavery would be permitted to expand into newly acquired Western territory destined to become states. Initially Congress had admitted new states into the Union in pairs, one slave and one free. This had kept a sectional balance in the Senate but not in the House of Representatives, as free states outstripped slave states in population. Thus, at mid-19th century, the free versus slave status of the new territory was a critical issue, both for the North, where 
Title: End of slavery in the United States of America
Content:  The civil war in the United States from 1861 until 1865 between states supporting the federal union ("the Union" or "the North") and southern states that voted to secede and form the Confederate States of America ("the Confederacy" or "the South"). The central cause of the war was the status of slavery, especially the expansion of slavery into newly acquired land after the Mexican–American War. On the eve of the Civil War in 1860, four million of the 32 million Americans (nearly 13%) were black slaves, mostly in the South. The practice of slavery in the United States was one of the key 

Question: what was the immediate catalyst to the civil war?
Answer: slavery.