These things are never decided by laws.
They are decided by facts and events - power, money, debt, violence, common purpose or disunity.
The Roman empire was united, until corruption, debt, inflation, decline and imperial overreach made people in the provinces decide they were better off out - and the emperors could do nothing about it.
Assuming Hillary Clinton defeats Trump, with the selection of liberal justices to the Supreme Court and the extension of the pervert revolution of Obama, secession may be the only means to save at least some of the states from absolute depravity and a dictatorship of the left. Whether the Constitution permits secession of a state is not relevant. That document has been a dead letter since the New Deal. Whether the failed Southern secession was due to a desire to preserve liberty or save slavery is not relevant. We are not talking about tariffs or human chattel but the prevention of a long era of tyranny as Russia suffered for over 70 years.