The Supreme Court held in the Prize Cases that the "so-called Confederate states" (to use their phrase) were in rebellion and that the president was authorized/required to put down the rebellion.
Walt
Grier cited "jure belli", the law of nations as the defense for Lincoln's actions. He punted on the legal issue of secession, stating that "[t]heir right to do so is now being decided by wager of battle. In Texas v White he held that the state of Texas was not a member of the Union.