By Daniel Patrick Moynihan

Harvard University Press. 211 pp. $22.50

With his curious ability to get things half right, Woodrow Wilson in 1919 foretold how the United States would change if it failed to join an international league. It would have to defend its interests on its own, he said, and that would oblige future presidents to be alert to the threat of war all the time, ready on a moment's notice to order whatever action might be necessary. "And you can't do that under free debate. You can't do that under public counsel. Plans must be kept secret." So a president might have to order our troops into a combat situation before asking the Congress or telling the public. To know when and how to act, our government would need to establish "a spying system." The activities of other nations would have to be monitored "by secret agencies planted everywhere." Moreover we would have to maintain "a great standing army" and a permanent industrial complex producing military supplies so that our equipment would always be "up to date."

Wilson was right about what would happen, but wrong about why. The CIA, the Defense Department, the Joint Chiefs of Staff, the staggering peacetime military budgets, the military-industrial complex and the tendency of our presidents to act first and ask our support later, all came about after we did join an international league, the United Nations.

They were the product of the internationalist 1940s rather than, as Wilson had supposed, of the isolationist 1920s. Their coming about had nothing to do with whether we joined a league; it had to do with whether we joined the world. We could have remained the country we used to be only by genuine isolationism: by withdrawing into the Western Hemisphere, if indeed that were possible.

It was our participating fully in world affairs -- which eventually we felt obliged to do -- that led our government to change in the ways that Wilson predicted.

In this erudite yet immensely readable brief history of American internationalism in the 20th century, with its far-ranging and provocative discourses on the issues to which that internationalism has given rise, Sen. Daniel Patrick Moynihan deplores these trends against which Wilson warned. He pictures an executive branch of government that, in the name of national security, routinely lies and cheats. What most arouses his anger is not that the president's men break the rules, but that they do not know that there are any rules.

Some of the laws they disregard, he tells us, are those enacted by Congress. But what especially draws his ire is their disregard of the rules of international law: of "the law of nations," as it was called in the 18th century and in the title of his book.

Much of international law, he reminds us, is now codified in treaties which the United States has signed; and, to that extent, by operation of the supremacy clause of our Constitution, it has become the law of the United States ("ARTICLE VI . . . all treaties made, or which shall be made, under the authority of the United States, shall be the supreme law of the land . . .").

He is not unmindful of the fact that most international agreements in fact are kept, notably the web of treaties and other international arrangements that govern everything from sending a letter or telegram to a foreign country to protecting migratory birds. This is the stuff of which everyday life is made; and even in approaching such great issues as the future of the Eastern European countries, we now think routinely in terms of international institutions, international cooperation and the negotiation and observance of international accords.

It is really our approach to international conflicts -- the situations in which international law is often disregarded -- of which Sen. Moynihan complains. He is no dotty idealist. He does not expect the Hitlers or Stalins of this world to be inhibited by the constraints of international law, nor would he bet the family farm on the certainty that foreign countries will always keep their pledged word. He is aware that in international conflicts, the decencies are among the first casualties. He knows that in the real world, national interests are of overriding importance, sometimes requiring even us to break pledges or violate laws.

His plea is that we should not do so lightly. To him, such recent American actions as the raid by U.S. troops on the residence of the Nicaraguan ambassador in Panama express nothing less than contempt for accepted standards of international conduct: standards that we ought instead to be trying to uphold in an effort to make the world at least a bit more civilized.

In the arena of world politics, where important interests collide and where stakes and passions can run high, the Law of Nations, which European Christians developed over the centuries in their dealings with one another, has at best a modest role to play. At times, Sen. Moynihan overstates it, ignoring the reasons why excessive faith in international law has been discredited over the course of the 20th century, and failing to focus on the special characteristics of European society that led the Law of Nations to take hold in an earlier age to an extent that it could not today.

But he performs a valuable service, not merely by posing the big questions that in the rush of everyday events we tend to forget, but by recalling us to a sense of what we ought to be. He takes the long view; he converses on easy terms, across the centuries, with the great thinkers of the past; and his discourse is informed by an exalted sense of what it is to be an American. The burden of his message is that we ought to play the game by the rules, not so much because we owe it to others as because we owe it to ourselves.

David Fromkin, a New York lawyer who did his postgraduate work in international law at the Institute of Advanced Legal Studies of the University of London, is the author of "A Peace To End All Peace: Creating the Modern Middle East, 1914-1922."