I'm going to give a very abbreviated summary of events leading up to the American Civil War.
Generally speaking, slavery was not considered a monumental issue amongst antebellum Americans prior to the Kansas-Nebraska Act, which shattered the previous status quo established by the Missouri Compromise and Compromise of 1850. The efforts of "fire eaters," especially Senator David Rice Atchison, resulted in the new doctrine of "popular sovereignty" to determine whether new states would allow slavery or not. This caused a civil war in Kansas and resulted in the Whig Party being disestablished in favor of the Republicans, whose primary platform issue was the long-term elimination of slavery by not admitting new slave states. Southerners associated this with abolitionism, and thus seceded from the Union when Abraham Lincoln won the election of 1860.
It's a myth that the South was losing influence with the federal government. The South had dictated the tariff policy of the nation since the repeal of the Tariff of 1828; the South had dictated the federal policy towards African-Americans in the Supreme Court; the South had dictated the admission of slave states after the Mexican-American War and in the Louisiana territories; et al. And now for the reasons why the American Civil War was entirely about slavery:
Argument #1: The "Cornerstone Speech" given by Confederate Vice President Alexander Stephens on March 21, 1861, contained this passage: "The new constitution has put at rest, forever, all the agitating questions relating to our peculiar institution African slavery as it exists amongst us the proper status of the negro in our form of civilization. This was the immediate cause of the late rupture and present revolution. Jefferson in his forecast, had anticipated this, as the 'rock upon which the old Union would split.' He was right."
Argument #2: From the South Carolina Declaration of Causes of Secession (1860): "We affirm that these ends for which this Government was instituted have been defeated, and the Government itself has been made destructive of them by the action of the non-slaveholding States. Those States have assume the right of deciding upon the propriety of our domestic institutions; and have denied the rights of property established in fifteen of the States and recognized by the Constitution; they have denounced as sinful the institution of slavery; they have permitted open establishment among them of societies, whose avowed object is to disturb the peace and to eloign the property of the citizens of other States. They have encouraged and assisted thousands of our slaves to leave their homes; and those who remain, have been incited by emissaries, books and pictures to servile insurrection. For twenty-five years this agitation has been steadily increasing, until it has now secured to its aid the power of the common Government. Observing the forms of the Constitution, a sectional party has found within that Article establishing the Executive Department, the means of subverting the Constitution itself. A geographical line has been drawn across the Union, and all the States north of that line have united in the election of a man to the high office of President of the United States, whose opinions and purposes are hostile to slavery. He is to be entrusted with the administration of the common Government, because he has declared that that Government cannot endure permanently half slave, half free, and that the public mind must rest in the belief that slavery is in the course of ultimate extinction."
