Under the constitutional scheme, to declare war was to define war. This operative principle had little to do with the nuances of conceptualizing war (that would come much later), and everything to do with the formalities of declaring war. That is, war was whatever Congress said it was. Thus, writing in 1801, Chief Justice Marshall explained in Talbot v. Seeman: "The whole powers of war being by the constitution of the United States, vested in congress, the acts of that body can alone be resorted to as our guides in this [wartime seizure case]." ²⁰

Chief Justice Marshall's maxim and Congress's notion of war were linked, of course, to the eighteenth and early nineteenth centuries' understanding of "war" as a state of armed hostilities between two or more sovereign nations created by a formal declaration of war and ended by an official declaration of peace, and "limited only by the laws and usages of nations."²¹ It is much that same determinate meaning of "war" that governed when President Woodrow Wilson outlined the case for declaring war against Germany in a speech to the joint houses of Congress on April 2, 1917; when Congress declared war four days later; when Congress made war a predicate to prosecution under Section 3 of the Espionage Act of 1917 (amended by the Sedition Act of 1918);²² and when Justice Holmes penned his opinion in Schenck v. United States.²³

Since Holmes's day, the definition of "war" (in contrast to the issue of who is constitutionally authorized by the text to initiate hostilities) has been transformed by America's involvement in armed conflicts during the late twentieth and early twenty-first centuries.