“The Civil War, from the Union side, was about preserving the Union, whole and undivided, as our parents and grandparents had passed it to us.”
No voluntary union of free states can possibly be maintained by force of arms. If it requires force, the Union is no more.
No voluntary union of free states can be maintained if some states believe they are more equal or have more rights than the others, either.
Besides, no force was used until the confederacy resorted to war to further their aims.