WASHINGTON -- The president's authorization of domestic surveillance by the National Security Agency contravened a statute's clear language. Assuming that urgent facts convinced him that he should proceed anyway and on his own, what argument convinced him that he lawfully could?
   
 Presumably the argument is that the president's implied powers as commander in chief, particularly with the nation under attack and some of the enemy within the gates, are not limited by statutes. A classified legal brief probably makes an argument akin to one Attorney General John Ashcroft made in 2002: ``The Constitution vests in the president inherent authority to conduct warrantless intelligence surveillance (electronic or otherwise) of foreign powers or their agents, and Congress cannot by statute extinguish that constitutional authority.''

     Perhaps the brief argues, as its author John Yoo -- now a professor of law at Berkeley, but then a deputy assistant attorney general -- argued 14 days after 9/11 in a memorandum on ``the president's constitutional authority to conduct military operations against terrorists and nations supporting them,'' that the president's constitutional power to take ``military actions'' is ``plenary.'' The Oxford English Dictionary defines ``plenary'' as ``complete, entire, perfect, not deficient in any element or respect.''

     The brief should be declassified and debated, beginning with this question: Who decides which tactics -- e.g., domestic surveillance -- should be considered part of taking ``military actions''?

     Without more information than can be publicly available concerning threats from enemies operating in America, the executive branch deserves considerable discretion in combating terrorist conspiracies utilizing new technologies such as cell phones and the Internet. In September 2001 the president surely had sound reasons for desiring the surveillance capabilities at issue.

     But did he have sound reasons for seizing them while giving only minimal information to, and having no formal complicity with, Congress? Perhaps. But Congress, if asked, almost certainly would have made such modifications of law as the president's plans required. Courts, too, would have been compliant. After all, on Sept. 14, 2001, Congress had unanimously declared that ``the president has authority under the Constitution to take action to deter and prevent acts of international terrorism,'' and had authorized ``all necessary and appropriate force'' against those involved in 9/11 or threatening future attacks.

     For more than 500 years -- since the rise of nation-states and parliaments -- a preoccupation of Western political thought has been the problem of defining and confining executive power. The problem is expressed in the title of a brilliant book, "Taming the Prince," by Harvey Mansfield, Harvard's conservative.

     Particularly in time of war or the threat of it, government needs concentrated decisiveness -- a capacity for swift and nimble action that legislatures normally cannot manage. But the inescapable corollary of this need is the danger of arbitrary power.

     Modern American conservatism grew in reaction against the New Deal's creation of the regulatory state, and the enlargement of the executive branch power that such a state entails. The intellectual vigor of conservatism was quickened by reaction against the Great Society and the aggrandizement of the modern presidency by Lyndon Johnson, whose aspiration was to complete the project begun by Franklin Roosevelt.

     Because of what Alexander Hamilton praised as "energy in the executive,'' which often drives the growth of government, for years many conservatives were advocates of congressional supremacy. There were, they said, reasons why the Founders, having waged a revolutionary war against overbearing executive power, gave the legislative branch pride of place in Article I of the Constitution.

     One reason was that Congress' cumbersomeness, which is a function of its fractiousness, is a virtue because it makes the government slow and difficult to move. But conservatives' wholesome wariness of presidential power has been a casualty of conservative presidents winning seven of the last 10 elections.

     On the assumption that Congress or a court would have been cooperative in September 2001, and that the cooperation could have kept necessary actions clearly lawful without conferring any benefit on the nation's enemies, the president's decision to authorize NSA's surveillance without the complicity of a court or Congress was a mistake. Perhaps one caused by this administration's almost metabolic urge to keep Congress unnecessarily distant and hence disgruntled.

     Charles de Gaulle, a profound conservative, said of another such, Otto von Bismarck -- de Gaulle was thinking of Bismarck not pressing his advantage in 1870 in the Franco-Prussian War -- that genius sometimes consists of knowing when to stop. In peace and in war, but especially in the latter, presidents have pressed their institutional advantages to expand their powers to act without Congress. This president might look for occasions to stop pressing.