IF YOU KNOW . . . not the enemy," the famous Chinese strategist Sun-tzu wrote over 2,500 years ago, "for every victory gained, you will suffer a defeat." Victory gained was not a problem on the battlefields of Vietnam. "The historic fact to emerge was that the U.S. Army during its entire stay, from 1965 to 1973, did not lose a single important battle," writes Douglas Pike, now director of the Indochina Studies Program at the University of California, Berkeley, in his important new book, PAVN: People's Army of Vietnam. "It was a record unparalleled in the history of modern warfare. Had the Vietnam War been another conventional war, had it been decided on the basis of past wars, it would have been over by mid-1963 with the defeat of the communist forces."

But despite its tactical successes it was the United States, not the communist forces, that ultimately went down to strategic defeat. How could that happen? In the first place, we "knew not the enemy." As Pike points out, "Throughout the entire Vietnam War, in and out of the U.S. government, such matters as the composition and mindset of the PAVN High Command, the operational code used in military decision making, an assessment of Hanoi's strengths and vulnerabilities, even PAVN's grand strategy, went virtually unexamined."

Secondly, Pike believes, we were faced with "a strategy for which there is no known proven counterstrategy." Called dau tranh, roughly translated as "struggle," it had two basic forms which operate in concert to defeat the enemy: dau tranh vu trang (armed struggle) and dau tranj chinh tri (political struggle). "Armed struggle" involved the use of military force by regular as well as irregular (i.e., guerrilla) troops. As long as American forces were in Vietnam, "armed struggle" was a disaster for the North Vietnamese, costing them (by their own admission) the lives of more than a half-million of their soldiers to no appreciable battlefield gain. But as Pike emphasizes, to counter the enemy strategy of armed struggle alone does not win the war. "To win one has to defeat both armed dau tranh and political dau tranh."

One important element of political struggle was dich van, or "actions among the enemy." Dich van "sought to do battle with America on its home ground, not with guns but with weapons of perceptional obfuscation." It was a "communicational effort to reshape the semantic environment." Pike sees this as peculiar to the Vietnam War, but in the 1920s and 1930s H.L. Mencken made almost exactly the same observations about the successful World War I British propaganda campaign to turn America against the "Hun."

Although certainly one of America's foremost experts on Vietnam, Pike himself acknowledges that he is not a military strategist. And because he is not a strategist he fails to see that "political struggle" is hardly a new strategism. A century-and-a-half ago the Prussian military theorist Carl von Clausewitz identified public opinion and the community of interest among allies as primary centers of gravity in warfare, equally as important as destroying an enemy's army or seizing his capital.

It was not so much that North Vietnam's dich van offensive was successful in subverting American public opinion and breaking the U.S.-South Vietnamese alliance as it was that America's psychological defenses against such an onslaught were so weak. As former secretary of state Dean Rusk has observed, the U.S. government deliberately opted to fight the Vietnam War "in cold blood," and specifically avoided rallying public opinion. It wasn't North Vietnamese propaganda or even the antiwar movement that turned the American people against the war. It was their all-too-correct perception by fall 1967 that the government didn't know what it was doing. As Clark Clifford found when he took over as secretary of defense after the enemy's 1968 Tet Offensive, even at that late date the United States had no plan to end the war, and no concept of victory. With such a state of affairs, almost any enemy strategy would have been effective.

But there is much more to PAVN: People's Army of Vietnam than Pike's assessment of North Vietnamese strategy. It is without question the best work available on what is now the world's third largest military force. Pike's analysis of what he labels "the Prussians of Asia," a country which today "has one of every three of its male adults in the military service or retired from it" is exceptionally well done. So is his prognosis of where this "modern-day Spartan social system" is going. As Pike sees it, there is a Poland in Vietnam's future -- "Marxist 'fascism,' defined as Marxism as interpreted, and run, by the Marxist military."

That is hardly what the North Vietnamesee soldiers were fighting for when they marched off to war a quarter-century ago. They may have won the war, but as things look now, they almost certainly have lost the peace.