Washington ? The fury that the American experience in Vietnam still generates in U.S. politics and thought resembles the war itself. It is out of all proportion to reality. The tawdry campaign-ad battles of 2004 are a costly distraction from the underlying challenges that will change history.

Southeast Asia was not a turning point in the Cold War. America’s defeat was — in military, strategic and economic terms — absorbed and reversed less than a decade after the fall of Saigon. The capitalist dominoes of Thailand and the Philippines have had more influence on the communists of Hanoi than vice versa.

But Americans still have difficulty in adjusting to how relatively small the ripples sent out by that conflict are in world politics and strategy. Three decades after the fact, “Vietnam” is still metaphor and memory for Americans who lived through all that word has come to encompass.

We look instinctively to that experience for instruction even as we should be intellectually discounting its relevance to the challenges American society faces at home and abroad in the 21st century.

That is one price we pay for sinking so easily into the mud bath that campaign operatives of both parties have prepared by trying to segment and thus change the meaning of John Kerry’s sequential history as decorated war hero and lionized antiwar dissident.

Kerry was both, and is both today in some measure that only he genuinely knows. As Ward Just, the most thoughtful of the journalistic witnesses of the decisive phase of the Vietnam War, makes clear in his new novel, “An Unfinished Season,” war (and other traumatic) experiences this long after the fact are what those who underwent them make of them now. They are impossible to transmute, live again or escape.

That is, they cannot be reliably applied in isolated fashion as guides to future character or fate. Too much has come between. The Democrats erred in trying to reopen the book on Vietnam to one page only: Kerry’s film-clip stroll through the jungles in battle gear. The Republicans try to obliterate that page altogether.

The moral choice Kerry made in opposing the war soon after that walk — making at times incendiary accusations he now does not repeat — is as important and as worthy of examination. Taken together, both commitments present a man who learns from his experiences and throws himself passionately into new courses of action. This is not indecision or vacillation. It is perhaps an excess of decision.

The duality Kerry embodies is difficult to process for a nation that is uncertain what to make of America’s continuing sacrifices in lives and treasure in fighting insurgency and terrorism in Iraq and Afghanistan.

A cottage industry has sprung up to compare and contrast Vietnam and Iraq, usually from a predetermined point of view. Often ignored is the most crucial contrast, which lies here at home, where “Vietnam” did have enormous impact as the nation’s cultural, political and military establishments were shaken loose from their moorings and forced to accelerate a societal modernization that is now at its term.

Kerry’s conversion to dissident came as the American public was increasingly turning away from all of the justifications offered to explain the sending of a conscript army to Vietnam. By October 1967, the risk that the communists would establish beachheads in California if U.S. forces left Saigon was seen either as ludicrous or as acceptable in mainstream public opinion.

Few Americans today seem inclined to believe that a total U.S. retreat from the Middle East and Central Asia at this point is desirable or possible — not while al-Qaida and allied groups threaten to surpass the atrocities of Sept. 11, 2001, and vengeful Baathist remnants fight to regain Iraq. The danger to Americans at home is felt as real.

That is why Kerry continues to promise not to cut and run, and to discuss troop withdrawals only in conditional terms. That is why the Democrats put forward Kerry the warrior, not Kerry the dissident, as the person to beat George W. Bush. That is why Kerry and Bush owe the public real answers about defeating Islamic-based terrorism, not advertising fights over the past.

The dirty-tricks squads have failed to convince me that Kerry’s performance under fire in Vietnam is somehow less honorable than Bush’s National Guard service stateside during that conflict. But Kerry has not yet succeeded in offering the convincing alternative to Bush’s handling of the war against terrorism that would make these arguments politically irrelevant. That is where the hinge of this campaign rests as it heads for the Labor Day turn.

Jim Hoagland is a columnist for Washington Post Writers Group.