Can a state leave United States of America?

Answer 1

No it is illegal. See Texas v. White 74 US 700 Supreme Court 1869: 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.

---The Constitution provides a procedure for sucession NOT UNILATERALLY, but by consent of the other states through a constitutional convention. Revolution could be a means of achieving succession. This would be difficult as the Fedral Government would exercise its duty to suppress rebellion in any participating state.