"During the 20th century," this book contends, "aggressive Presidents and supine Congresses have transformed the President's constitutional authority to defend the nation against attack into a virtually unlimited power to initiate undeclared war and military hostilities." New theories therefore are needed to guide Congress, President, and courts in future struggles over the distribution of the war powers. White House spokesmen since the Truman administration have reiterated a constitutional theory that confers inherent power on the President to dispatch and commit armed forces without congressional approval or consultation. This tendency was not reversed by congressional attempts to limit presidential warmaking following the Vietnam War; it was encouraged by the Federal courts' position in Vietnam cases that only "prolonged, irreconcilable legislative-executive conflict should serve as an invitation to judicial intervention in war-powers controversies."

A major feature of the book is a thorough analysis of all the legal challenges to the President's conduct of the Vietnam War. The Vietnam cases are examined in light of British constitutional history, the framing, of the American Constitution, and judicial decisions from 1800 through the Korean War. This analysis furnishes the basis for the author's contention that the Supreme Court has led the nation into the "twilight zone of concurrent power"—encouraging "the legislature and the executive to fuse their separate powers of war and defense into a national war power whose only standard is the extraconstitutional one of success on the battlefield."

In the modern era of guerrilla wars, national liberation movements, and police actions, the author recognizes the inadequacy of traditional distinctions between defensive and offensive wars upon which the Framers of the American Constitution divided the congressional war powers from the office of commander in Chief. Keynes concludes that, although the courts can play a limited role in restraining presidential power to conduct undeclared war, only Congress can effectively limit the President's conduct by insisting on a prior consensus regarding military intervention.