In renewed pursuit of the things that divide Jimmy Carter and Ronald Reagan fundamentally in matters of foreign policy, we proceed now to Vietnam.

Not the war itself. But Vietnam as a learning experience. The utterly opposite way that Reagan and Carter look-back upon this country's Vietnam experience says almost everything about the wholly different way the two men see America's role in the world.

What's revealed are not differences in policies, exactly, or even in grand strategy, but a difference in attitude and approach (which actually may be a lot more reliable than campaign promises as a portent of future performance).

For Reagan, the Vietnam War was, in his now famous phrase, "in truth, a noble cause" -- a rhetorical excess that, given its context, got him more trouble than he deserved. Carter sees it, in retrospect, as an internal conflict into which the United States should never have allowed itself to be drawn in the first place.

That's their difference, in short. But there's a larger difference, and nowhere is it better defined than in a column by Walter Lippmann, written in February 1965. Then, the American involvement in Vietnam was still limited to military advisers and arms aid. The question of plunging in far more deeply with organized combat forces was still open.

What basically troubled Lippmann at that time was "how our intervention in the Second World War to defeat the Nazis and the Japanese became inflated into the so-called Truman Doctrine of the late 1940s, in which the United States said it was committing itself to a global ideological struggle against revolutionary communism."

Thinking that to be misguided, he was all the more skeptical about how Vietnam's fortunes could possibly "determine the fate of the world" or the position of the United States as a world power. "There are men saying today that the defense of Saigon is the defense of Hawaii," Lippmann wrote, disbelievingly. "For those who think that way, there is no stopping point between globalism and a retreat into our former isolation."

Ronald Reagan is one who thinks -- and talks -- that way. He has defended the Vietnam War not once, but repeatedly. "It's a lot easier and a lot safer," he said four years ago, to "counter the master plan of the communists for world conquest 8,000 miles away than to wait until they land in Long Beach."

Hence the "noble cause" -- "noble" not only in the sense of the sacrifice of those who fought it, but in the crucial context of a "global ideological struggle." To him, as he said in the same speech (to a convention of the Veterans of Foreign Wars two months ago), Vietnam was "a small country newly free from colonial rule [that] sought our help in establishing self-rule and the means of self-defense against a totalitarian neighbor."

Not surprisingly, the Reagan lesson drawn from Vietnam, as he preached it to the VFW delegates, was that "we will never again ask young men to fight and possibly die in a war our government is afraid to let them win."

The American failure in Vietnam, in other words, was an unwillingness to win. By the Lippmann test, it would seem to follow, Reagan is among those for whom there is "no stopping point between globalism and a retreat into our former isolation," no case for cautious, picking and choosing among communist pressure points for those where American counterforce can most usefully be brought to bear. He would draw lines, create defense alliances, defend staunch anti-communists, take no chances with leftist, Marxist-oriented revolutionaries.

For his part, Carter ranks his support for the Vietnam War (until 1971) close to the top of his list of things he wishes he had done differently. That's what he told Playboy magazine in a 1976 interview, which attracted more attention for what he said about lust (in his heart).

"The people," he said, "were tremendously misled about the immediate prospects for victory." At first he accepted the conventional wisdom that "we were protecting our democratic allies."

But in the 1960s, Carter added, he came to accept the view of the critics that "we ought not to be there, that we should never have gotten involved, we ought to get out." He just never had occasion as a state politician or a farmer, he explained, to speak out.

And so, Jimmy Carter has to be put down as a believer, with Lippmann, that between "globalism" and retreat to isolationism there are reasonable and realistic "stopping points" and that it is the business of statesmen to find them.

The Vietnam record is not offered here as a sure guide to how Carter or Reagan would respond to a particular communist challenge in a particular place. But it does suggest yet another fundamental difference in the predilections of the two men -- and another basic for choice. d