The supreme curt ruled that secession is unconstitutional in the case of Texas vs White, in which the state of Texas claimed that the bonds it held before the war were sold illegally by the state government under the Confederacy. The supreme court found in a 5-3 decision that unilateral secession by individual states is unconstitutional and thus the wartime Texan government to be illegitimate. That the union, once forged, is a perpetual entity. Once a state entered the union (and all states entered voluntarily btw), it cannot leave at its own discretion.