I won't attempt to predict the reaction of the President when the first state secedes within this decade. My guess is that he will offer a show of force, but before any blood is shed, he will let the state go in peace. Who will ever forget Boris Yeltsin's stand? One man faced down the Soviet Union's military might led by a column of tanks. How would it look for the United States to kill one sitting in the capital of the seceded state? How will it look to all Americans and the rest of the world for a President and the United States to declare war on a non-violent seceding state acting under the Declaration of Independence?
Is the United States prepared to be beseiged by terrorism of its own citizens making on a growing and unprecedented scale. My own bet is that the US will not use force or resort to war. If they can not contain the secession with local police/national guard without blood shed; I don't think they will resort to military action. If the seceding state used any show of force, violence or confiscation of US property or detained any US citizens against their will, it would be completely the opposite. The revolt would be ended with whatever military force was required.
The determinging factor will be the statements of the seceding state, that the action is not only non-violent but clearly peaceable without malice or infringement on the rights of any who did not wish to participate in the secession.