The Civil War was decided by force of arms rather than legal argument and legal decisions.

From a legal standpoint, the Civil War was brought on by the State of South Carolina et. al. attacking the U.S. Government. This is also known as treason, and the Civil War is more properly known as The War Of The Rebellion. The use of arms to protect and maintain the United States by force of arms was a perfectly legal response, and appropriate to the circumstances.

As for a strict vs. loose interpretation of the US Constitution, that's a whole other discussion.