Congress was not in session when Lincoln launched his war against the South and raised his own army with the help of Republican governors. When Congress was called into session in July of 1861, Lincoln had already suspended habeas corpus and arrested those he considered disloyal citizens, which included elected legislators.
Lincoln’s familiar opponent Chief Justice Roger B. Taney stated “that the president had the power to handle the insurrection, but he could not himself declare a civil war, establish a blockade, or confiscate ships of foreign neutrals.
Power was exercised by the chief executive under the municipal laws of the country “and not under the law of nations.” Only Congress could declare war, a US president is not a monarch whose word is law, and treason as defined in the Constitution “shall consist only in waging war against them,” the States.
Defining Insurrection and Enemies
“The Union hunted down more than privateers on the ocean; its navy also captured merchant ships – blockade runners or neutral carriers of contraband for the Confederacy. From the start of the war, the handling of these cases in district courts piqued public interest for what the rulings said about the enemy and confiscation of property.
By 1863 [the Supreme Court Prize Cases] rulings . . . revealed a shift in thinking taking place throughout the Union toward treating everyone as enemies in an area considered in insurrection, regardless of individually professed loyalties.
Lincoln acted with somewhat debatable constitutional authority . . . Capturing vessels and declaring blockades typically involve a war between nations. Only the Congress, not the president, has the power to declare war. But the legislative branch was not in session when war commenced and was not scheduled to gather until December 1861, so there was no congressional action on these issues until the president convened a special session in July.
Congress on July 13 passed what became recognized by the judiciary as the nearest thing to a declaration of war that could be found. The legislation allowed the president to collect duties on imports in the States mounting insurrection, to interdict all trade, and to seize vessels judged to be in violation of the statute. The language was incredibly turgid and legalistic: it was hardly an act designed to inspire a public, but it did concede to the president the ability to define an insurrection and decide on the measures to take against the Confederacy.
[Pennsylvanian and Democrat Supreme Court Justice Robert C.] Grier provided the administration with a victory. Writing the majority opinion . . . he reasoned that the president had the authority to act without legislative authority. It was his duty to defend the country against insurrection or invasion. “The President was bound to meet it in the shape it presented itself without waiting for Congress to baptize it with a name; and no name given to it by him or them could change the fact.”
[Grier further declared that] As long as people lived within regions considered by the president in rebellion against the government, they were enemies, although not foreigners. Grier reserved for the government the ability to prosecute traitors after the war.
Especially after Lincoln appointed Republican justices, the Supreme Court stood ready to support the president on issues concerning secession and war powers . . . and as far as the courts were concerned, no Unionists live in the Confederate States. Grier had made that clear, indicating that treason had become identified with individual persons than with territory.”
(With Malice Toward Some: Treason and Loyalty in the Civil War Era, William A. Blair, UNC Press, 2014, excerpts pp. 77-78)