The answer to the question, Why did the Confederate States fight the war?"  is glaringly obvious:  Self-defense. They fought to repel an armed invasion.  

There would have been no war had the Confederate States not declared independence, so the real question to address is "Why did the Confederate States declare independence (secede from the Union)?"

There were certainly other factors which contributed to the climate of distrust and war, among them the issue of slavery and the slavery status of the new western states and territories.  But as discussed in the previous section, the Northern states intended to hold the southern states for economic reasons.  The South wanted independence for economic reasons.  Follow the money!

Simply put, it was a tax revolt.  In the early years of the American Republic, the federal government lacked the power to fully enforce its revenue laws.  That was rapidly changing in the early to mid-19th century.

South Carolina defied the United States in 1832 in protest of the Tariff of Abominations(1828).  Senator John C. Calhoun argued that the vast majority of federal revenues were paid by the South, yet the vast majority of the expenditures were for the benefit of Northern industry---driven, of course, by the Northern majority in Congress.  South Carolina nullified the tariff in 1832 , that is, declared the tax unconstitutional and therefore void in South Carolina.  President Andrew Jackson threatened to use force to collect the tax.  Only the Great Compromise of 1833 averted war.

The Democrats gradually reduced the unfair tariffs through the 1840s and 1850s, but opposing forces were brewing.  The Democratic Party split in 1860 (Douglas and Brickinridge) and a new Constitutional Union Party emerged.  The result was that Lincoln won with only 40% of the popular vote.  Even without the split, Lincoln would have won in the Electoral College due to the advantage of the populous North. The North finally had a completely sectional (Northern) party in control.

Radical abolitionists from the North had no doubt electified the climate for war with their hateful rhetoric and constant impugning of the character of slave-owners and southerners in general.  There were abolitionists in the South, too, but they fell silent once John Brown and other Northern-sponsored terrorists began their campaigns of violence and their attempts to incite violence in the South.  Abolition was merely an irritant and not a real incitement to war.  Republicans used the abolitionists as "useful idiots" to incite Northern distrust and anger towards the South, but abolition was not the driving concern for those controlling the northern Republican party.  Political dominance and economic exploitation via tax policy was their goal.  Now that the Northern Republicans held the power, they were determined to keep it