Predictably, then, the American political and legal system has merged over time the constitutionally divided war powers into a unitary national war power that often enables the President to wage "wars" not declared by Congress.⁴² "During the twentieth century," Keynes summarizes, "aggressive Presidents and supine Congresses have transformed the President's constitutional authority . . . . With the federal judiciary's blessing, Congress and the President have fused their respective and constitutionally distinct powers into a national war power that the commander in chief exercises with very few limitations."⁴³

Several historical examples help to illustrate this point:

Civil War Era: For any variety of political and prudential reasons, the Supreme Court (5-4) upheld President Abraham Lincoln's broad assumption of wartime powers when he imposed a naval blockade on Confederate ports that Congress retroactively sanctioned three months later. Although Justice Robert Grier's opinion for the majority in The Prize Cases ⁴⁴ acknowledged Congress's power to declare a national war, it nonetheless recognized that the President need not wait for express legislative authority before engaging in offensive and defensive emergency measures to suppress insurrections.⁴⁵ In effect, The Prize Cases ushered in a regime in which the separation-of-powers principle could be satisfied by ex post facto congressional ratification of unilateral executive initiation of hostilities, at least in emergency situations that could not await formal congressional approval.⁴⁶

Vietnam Era: America's military intervention in Southeast Asia was "the longest undeclared war in U.S. history, costing well over $100 billion and 360,000 American dead and wounded." ⁴⁷ Only after Harry S. Truman and Dwight D. Eisenhower sent economic assistance and military advisers to the French in Indochina, after John F. Kennedy escalated the number of military advisers and assigned them combat roles, and after Lyndon B. Johnson ordered the Navy to retaliate against the North Vietnamese in response to their alleged bombing of U.S. ships in the Gulf of Tonkin did Congress issue a resolution authorizing future Executive action. The 1964 Gulf of Tonkin Resolution ⁴⁸ was taken by the Executive to sanction a full-scale war in Vietnam. During that era, the federal judiciary invoked justiciability doctrines, such as standing and political question, to dispense with challenges to the constitutionality of the President's actions in the Vietnam conflict. ⁴⁹