This section is from the book "The Constitutional Law Of The United States", by Westel Woodbury Willoughby. Also available from Amazon: Constitutional Law.
That, under our Constitution, the United States may begin war against a foreign country only by a declaration issued by Congress has never been disputed, the Constitution expressly providing that Congress shall have the power to declare war. That a foreign nation, or insurrectionary body of citizens, may by invasion of the United States or by other acts bring about a condition of affairs which will warrant the President in declaring, in advance of congressional legislation, that a state of war exists, was asserted by the Supreme Court in the Prize Cases.47 After very properly holding that a public war may exist between a State and its rebellious citizens as well as between independent nations, the court say: "A civil war is never solemnly declared; it becomes such by its accidents - the number, power, and organization of the persons who originate and carry it on. "When the party in rebellion occupy and hold in a hostile manner a certain portion of territory; have declared their independence; have cast off their allegiance; have organized armies; have commenced hostilities against their former sovereign, the world acknowledges them as belligerents, and the contest a war."
This is true enough as to foreign powers. All nations have the power and right, usable at their discretion, in the case of a civil contest in another State, to determine, each for itself, whether the struggle is to be treated as a war and the parties to it as belligerents. They are not bound by the action which the State concerned may take or has taken. Nor, on the other hand, is that State bound by the action of foreign States. It may continue to treat as rebels the insurgents who have been recognized as belligerents by foreign powers, and it would, therefore, seem that, in the United States, from the constitutional viewpoint, it should lie with the war-declaring power, that is, with Congress, to determine when the civil struggle should be recognized as a war. In advance of such recognition the executive would have the authority to use the entire force of the nation in the enforcement of its laws, as, in the case of an invasion, or other attack by a foreign power, but, it would seem, he should not be given the power to do more than this and by his own ipse dixit declare that a public war exists. The court in the Prize Cases say: "If a war be made by invasion of a foreign nation, the President is not only authorized but bound to resist force by force. He does not initiate the war, but is bound to accept the challenge without waiting for any special legislative authority." This also is quite true. As between the United States and a State thus attacking it, and as concerns neutral States, a war may thus be brought into existence without a declaration. But from this it does not necessarily follow that as concerns our citizens the legal rights and responsibilities attendant upon a state of war may be brought into force without the action of the constitutional law-declaring power. However, the court in the Prize Cases say: "Whether the President in fulfilling his duties, as Commander-in-Chief, in suppressing an insurrection, has met with such armed hostile resistance, and a civil war of such alarming proportions as will compel him to accord to them the character of belligerents, is a question to be decided by him, and this court must be governed by the decisions and acts of the Political Department of the government to which this power was intrusted. He must determine what degree of force the crisis demands. The proclamation of blockade is, itself, official and conclusive evidence to the court that a state of war existed which demanded and authorized a recourse to such a measure, under the circumstances peculiar to the case." 48
47 2 Black, 635; 17 L. ed. 459.
48 In the Prize Cases four justices, including Chief Justice Taney, dissented, the ground of this dissent being in a considerable measure that indicated by the author.
The powers of Congress with reference to the prosecution of a war, and some of the legal incidents to a state of war are discussed in later chapters.49