We may dismiss out of hand Paul Hoerner's statement that the Confederacy's secession was illegal. It was an open question in antebellum America; secession was widely though not universally considered constitutional. President James Buchanan, who disfavored breaking the Union up, also stated that Congress had no authority to keep the Union by force.
A century later President Dwight Eisenhower noted “that at the time of the War between the States the issue of secession had remained unresolved for more than 70 years.” (It should go without saying that superior brute force didn't resolve the issue of secession, other than that might makes right.) Of course, President Jefferson Davis thought secession entirely constitutional.
Hoerner is right that the Confederacy's leaders used slavery as the excuse for secession, but the bulk of the monuments in dispute now are of their generals and soldiers. Since these are memorials to the men who fought for their homes against the North, their purpose has nothing to do with justifying slavery. Thus the Confederate flag represents the valor of those who thought they were fighting for independence as the colonists did against England, and not slavery.
One's view of secession depends heavily on his view of the Constitution as either a compact between states or a consolidated government. When the Constitution was taken seriously so were states' rights and the power to secede (which power New York, Rhode Island and Virginia claimed in their ratifying conventions by stating their right to take back everything delegated to the federal government, if necessary). In our modern hypocrisy, however, we decry secession for us while encouraging it throughout the world.
Ross Nelson, Casselton