For the first time in nearly two decades, war with the Soviet Union has turned from seeming theoretically possible to seeming actually possible -- and not just cold war but hot war, a shooting war, even a nuclear war. The phrase "World War III" is on many tongues. The question is being increasingly asked, "How serious is this? Are we going to war?"

The new phenomenon -- war talk, war feeling -- is diffuse. At one level, it exists as a kind of explanation or metaphor for the current tension and for the Carter administration's responses to it. Jimmy Carter, for example, suggested again in his State of the Union address that world peace is now in greater jeopardy than at any time since World War II. Author Richard Barnet, who with presidential aide Zbihniew Brzezinski has a better record of anticipating Soviet aggressiveness than just about anybody, wrote in this newspaper last Sunday: "The world seems closer to a major war than at any time since the 1930s." Hugh Sidey writes in Time about Carter's emergence: t"There are some good signs. . . .He talks about power and the possibility of war as he used to talk government reorganization and revenue sharing."

At a second level, war talk exists as a budding psychological reality, on the way to becoming a political reality, with which public figures think they must deal. Thus "White House officials," who presumably are not displeased to see the public responding to Carter's new appeals, felt constrained this week to identify and assuage "public fears that the United States is heading for a war with the Soviet Union," as The Post wrote. Carter appeared to have a similar balancing purpose in his message Wednesday night.

On a third level, war talk now has an undeniable reality in political and policy debate. Carter chose the less alarming euphemism, "use of military force," in stating on Wednesday how th e United States would react to a Soviet attempt to "gain control of the Persian Gulf region." Others are more explicit. Henry Kissinger, after telling The Wall Street Journal that "the longer we wait . . . the greater the danger of war," added: "But World War III is certainly not inevitable if we act firmly and wisely." The New Republic suggests we prepare to seize Iran's port of Chah Bahar, and much else. Columnists like George Will and Joseph Kraft are also into specific deployments.

Well, it is early in our talk about war, and none of us has gotten a firm handle on it. Still, the nature of a new conflict, as it has been discussed so far, has been notable mild. You might think the term World War III would be reserved for major, if not all-out nuclear, war between the great powers. In fact, the understood footnote on its current usage suggests the more reassuring prospect that "World War III" will involvle not much more than some heavy bumping and bruising by conventional forces on turf far from home. It is for that sort of war that we are now ginning up new bases, a larger naval presence, a rapid depolyment force, and so on.

The unstated premise is that we will "win," although the concept has not yet been precisely defined -- and although much has changed since the last time a Soviet-American war was regarded as a live option. The notion of an unsatisfactory outcome is now dangled chiefly as a specter to stimulate us to redouble our military preparations and stiffen our resolve.

War is being presented as something important, ominous, strategically and politically salvaging, energizing, perhaps even a bit cleansing -- but not particularly bloody, not destructive to us or our interests, not really likely to get out of hand, not something to give pause to a reasonable nation if it felt it had no choice.

Sometimes it seems what people who talk about war have in mind is not war, which can of course be messy, costly and disastrous on a national as well as a human scale, but simply a readiness to countenance war. It is as though a projection of will is the essential quality, and that can be achieved by, well, standing there and emanating will and if necessary, by conducting a surgical operation or two on the littoral of the Arabian Sea.

In brief, we are still in the chattering stage, still in our armchairs. I think that beforoe we can expect others to find our warnings of war credible -- and this must be our central intent -- we have to show we understand what serious business war really is.