Did slavery initiate the Civil War?

The short answer to this question is no. The Civil War's immediate cause was the Confederate attack on Fort Sumter in April of 1861. A more complex answer would be that the Civil War's causes lay in serious, possibly even irreconcilable political differences between the states of the Deep South and northern states. These differences had their roots in the institution of slavery, and indeed reached their fiercest pitch during debates over whether slavery should be allowed to expand into western territories. They culminated with the secession of the seven Deep South states (South Carolina, Alabama, Mississippi, Texas, Florida, Georgia, and Louisiana) after Abraham Lincoln, whose position was that slavery should not be allowed to expand any further than its present borders, was elected in November of 1860. It was secession which brought the Confederacy and the federal government to the situation they faced at Fort Sumter. Many of the departing states made it quite clear that the defense of slavery was the most important of their reasons for leaving the Union. South Carolina's "Declaration of Causes," issued by the secession convention, is one example:

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...[H]e 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. 

The Declaration, which was fairly typical of those issued by the secession conventions, went on to decry the fact that in some northern states, black men were allowed to vote. So while slavery did not start the Civil War, the issue of slavery was foremost among the causes for secession among the states who first left the Union.

Was slavery the primary cause of the Civil War?

Most historians would agree that slavery was the primary cause of the Civil War, because the issue of slavery lay just beneath all of the more specific issues that led to secession.

After the war, Southerners would try to distance themselves from slavery as a cause of the war, but during the secession crisis that followed Abraham Lincoln's election, proponents of disunion openly said that they wished to leave the Union to protect slavery. In their "Declaration of Causes" of secession, the secession convention in Georgia pointed to "...numerous and serious causes of complaint against our non-slave-holding confederate States with reference to the subject of African slavery." Most of the other Deep South states said something similar. The first seven Confederate states left the Union because they feared that the federal government under Lincoln and the Republicans would undermine slavery in the South, or perhaps more accurately, that they would refuse to accede to Southern demands on slavery as they had done (with the exception of a few issues) in the past. So the direct cause of the war was the secession of the Southern states, and the Confederate decision to fire on Fort Sumter in April of 1861. But the cause of secession was slavery. From a Union perspective, the purpose of the war would change over time from preserving the Union to ridding it of slavery. As James MacPherson, perhaps the most respected living Civil War historian recently said in an interview about the causes of slavery:

Probably 90 percent, maybe 95 percent of serious historians of the Civil War would agree on the broad questions of what the war was about and what brought it about and what caused it...which was the increasing polarization of the country between the free states and the slave states over issues of slavery, especially the expansion of slavery.”