Lincoln's First Inaugural Address

This seems like the most cogent argument against permitting the Southern States from succeeding in leaving the Union.

"I hold that in contemplation of universal law and of the Constitution the Union of these States is perpetual. perpetuity is implied, if not expressed, in the fundamental law of all national governments. It is safe to assert that no government proper ever had a provision in its organic law for its own termination... If destruction of the Union by one or by a part only of the States be lawfully possible, the Union is less perfect than before the Constitution, having lost the vital element of perpetuity."

Lincoln's First Inaugural Address
March 4, 1861