The root cause of the Civil War is a controversial topic in Charleston.

Amateur historians passionately argue whether the main cause was the South's commitment to maintain slavery or whether Southern states were rebelling against an intrusive federal government.

College of Charleston history professor Bernard Powers acknowledges the popular debate but says there's no doubt the war started over slavery.

"I'm going to start out with a controversial statement," Powers said during a lecture at a Charleston County Public Library branch Tuesday. "Slavery was at the heart of the coming of the Civil War."

Powers spoke to about 20 people at the John L. Dart Library on King Street. His lecture, called "A Nation Asunder: Slaves, Free Blacks and the Origins of America's Civil War," is one of several talks at local library branches this month. The talks are part of the Lowcountry Civil War Sesquicentennial Commemoration, which continues through April 2015. The commemoration is coordinated by the Fort Sumter-Fort Moultrie Historic Trust to mark the key events of the Civil War 150 years later.

"It's a real opportunity for Charleston and the rest of the nation to take stock of the meaning of the Civil War," Powers said of the commemoration.

Among Powers' arguments that slavery was the main cause of the Civil War, he cited South Carolina's declaration of reasons for secession in 1860. The document complained that the federal government was no longer enforcing the Fugitive Slave Act, which required federal troops with to help return runaway slaves to their masters.

Abraham Lincoln opposed expanding slavery into new territories. Whether to allow the expansion of slavery was a key issue in his presidential campaign just before the war.

On the other hand, the Supreme Court's 1857 Dred Scott decision said a man should be free to transport a slave, being his property, to any territory. "This has everything to do with the coming of the Civil War," Powers said.

South Carolina seceded immediately after Lincoln was elected in 1860, and the war was on.