The U.S. Constitution was itself an act of secession

Article XIII of the Articles of Confederation provided as follows:

Every State shall abide by the determination of the United States in Congress assembled, on all questions which by this confederation are submitted to them. And the Articles of this Confederation shall be inviolably observed by every State, and the Union shall be perpetual; nor shall any alteration at any time hereafter be made in any of them; unless such alteration be agreed to in a Congress of the United States, and be afterwards confirmed by the legislatures of every State.

That is, the union of thirteen states formed under the Articles was to be permanent, and said union could only be altered by agreement of all thirteen states.

Yet Article VII of the Constitution of the United States provided as follows:

The Ratification of the Conventions of nine States, shall be sufficient for the Establishment of this Constitution between the States so ratifying the Same.

That is, the authors of the Constitution explicitly agreed to ignore the law requiring a permanent union among the thirteen states under the Articles of Confederation; further, they provided that if only nine of the thirteen states agreed, those nine would form a new confederation among themselves, thereby seceding from the permanent union formed under the Articles.

Those who claim that secession is an illegal or immoral act must therefore agree that the formation of the second United States of America under the Constitution was an illegal or immoral act, for the second United States were formed through an act of secession.