What caused the Civil War? This is one of the thorniest questions in American history. But one thing is for sure: Abraham Lincoln did not declare war on the Confederate states to end the practice of slavery [source: Loewen]. He declared war because the Confederate states were attempting to secede and he wanted to save a unified USA.
Why did the Confederate states want to secede from the Union? Well, slavery for one thing. Wait ... didn't we just say the Civil War wasn't about slavery? No, we said that Lincoln didn't fight the Civil War to end slavery. The South was sick of federal laws that restricted the movement of its slaves in the North [source: Lichtman]. The decision to secede from the Union was to free itself of federal interference in the slave trade.
Let's let Lincoln himself explain his motivation for going to war, as written in a letter to the New York Tribune in 1862 [source: Loewen]:
If I could save the Union without freeing any slave, I would do it; and if I could save it by freeing all the slaves, I would do it; and if I could save it by freeing some and leaving others alone, I would also do that.
In his personal beliefs, of course, Lincoln was strongly antislavery. When he finally signed the Emancipation Proclamation in 1863 — three years into the war — it rallied the Northern forces around the abolitionist cause and gave the war effort a moral dimension. This is why most of us think the Civil War was always fought to end slavery, because that's exactly what it did.