In 1776 thirteen British Colonies in North America declared their independence and they separated themselves from their mother country,
In the Declaration of Independence colonies asserted that they had a natural right secede to secede.
Great Britain refused to accept that they had a lawful right to secede and engaged the colonies in bloody war to prevent their independence.
With the election of Abraham Lincoln in 1860 eleven southern states exercised their right to secede from following the precedence established in 1776.
In order to prevent their secession, Abraham Lincoln and preserve the Union he instigated the War of Northern Aggression.
In the Declaration of Independence, Thomas Jefferson wrote that when in the opinion of the people a government becomes tyrannical it is the right to the people to alter or abolish it.
Rather than attempting to alter or abolish the government the southern states decided to exercise their right to peacefully withdraw from the Union.
If it was lawfully for the 13 colonies to secede, why would the secession of 11 southern states be unlawful?
If you believe that the victory of the Northern States in the Civil War settle the issue of the secession, you must accept the idea that might makes right.