Several posters in the thread also concurred that the act was not illegal and additionally that it was a core principle of the United States to have the right to secede. Is this a widely held view among historians as well?
The Constitution does not include an 'escape clause' or any means to legally secede or dissolve the Union.
Furthermore, Article 1, section 10 states:
No State shall enter into any Treaty, Alliance, or Confederation; grant Letters of Marque and Reprisal; coin Money; emit Bills of Credit; make any Thing but gold and silver Coin a Tender in Payment of Debts; pass any Bill of Attainder, ex post facto Law, or Law impairing the Obligation of Contracts, or grant any Title of Nobility.
Due to the supremacy clause, no State can pass a law that would allow it to contradict the Constitution, or Federal law. The only way to legally secede would be to pass a Constitutional Amendment that granted that power to the states first.
In 1869, in Texas vs White (which came about over a war-time bond issue), the Supreme Court ruled:
When, therefore, Texas became one of the United States, she entered into an indissoluble relation. All the obligations of perpetual union, and all the guaranties of republican government in the Union, attached at once to the State. The act which consummated her admission into the Union was something more than a compact; it was the incorporation of a new member into the political body. And it was final. The union between Texas and the other States was as complete, as perpetual, and as indissoluble as the union between the original States. There was no place for reconsideration or revocation, except through revolution or through consent of the States.
Had Davis been tried for treason, he would have appeared in a Republican dominated court, likely resulting in an easy conviction. The Constitution defines treason as:
levying War against them, or in adhering to their Enemies, giving them Aid and Comfort.
Ignoring the entire rest of the Civil War, just the bombardment of Ft. Sumter would likely have been enough for a treason conviction. Davis was indicted for treason, but received clemency as part of Andrew Johnson's general grant of amnesty in 1868.