Brussels — With the end of the fifth round of superpower strategic arms control negotiations Thursday, the world enters an uncharted period in which:
* There are no nuclear arms control negotiations going on for the first time in two years.
* East-West relations - it is generally agreed - are at their worst in two decades.
* There is uncertainty about who is actually making decisions in the Kremlin.
NATO foreign ministers are therefore wrestling over how to proceed in this new period at their regular winter session in Brussels yesterday and today.
Chief Soviet negotiator Viktor Karpov left the superpower strategic arms reduction talks (START) in Geneva Thursday, saying that no date had been set for their resumption. This move followed Soviet ''discontinuation'' of the Euromissile arms control talks in Geneva in late November. In the past two weeks , Moscow has said repeatedly that it will not resume Euromissile talks until the NATO missile deployments beginning this month are revoked.
For now the NATO reaction is low key. The West is not treating cessation of the arms control talks as a crisis. It chides Moscow for suspension of the negotiations, but it argues that logically the Soviet Union will have to return to the negotiating table eventually.
The NATO foreign ministers are therefore united in opposing any new concessions by the West now in arms control. They believe they have basically proved the alliance's resolve and political will in beginning as scheduled the deployment of 572 intermediate-range Pershing II and cruise missiles between now and 1988. They think the Soviet Union has lost its four-year fight to turn European public opinion against the deployment and make them politically impossible.
At the same time, however, some of the allies - especially West Germany - are keen on keeping all the other channels for East-West contact as open as possible. This is aimed both at allowing the Soviet Union some face-saving way to return to arms control in the future - and at convincing European public opinion that it is only the Kremlin that is being obdurate at this point.
West German Foreign Minister Hans-Dietrich Genscher keeps saying the West must continue its own initiatives and not just react passively to the next Soviet initiative for resuming negotiations, whenever it comes. The West Germans have recently been joined in this view not only by Canadian Prime Minister Pierre Trudeau, but also - and more strikingly - by British Prime Minister Margaret Thatcher.
The immediate focus of this channel dredging, the West Germans contend, should be the conference on disarmament in Europe that will open in Stockholm next month. With the strong encouragement of the West Germans, US Secretary of State George Shultz has been saying in the past week that he would be ready to attend the Stockholm opening himself.
A meeting between Mr. Shultz and Soviet Foreign Minister Andrei Gromyko in Stockholm - and such is the clear intent of the West Germans - would be the first time the two superpowers have met on that high a level in a year.
Driving the West German urgency is a visceral concern that a revival of East-West negotiations and dialogue may not be as foreordained as the Bonn government thinks it is. To be sure, Moscow has as strong an interest in arms restraint and stabilization of deterrence as does the West, in terms of both economic burdens and sheer biological survival.
But there is a nagging worry that crisis in the world - especially in the Middle East, but possibly also in Latin America - could drag the superpowers into confrontation that neither might desire rationally.
This is especially troubling in a period of uncertainty in the Kremlin. Some Western Kremlinologists are asking whether the cumulative bad feeling between East and West might now bring Moscow to a theoretical rejection of the late President Leonid Brezhnev's detente.
Until now - whatever the superpower clashes in practice - the Kremlin has retained Mr. Brezhnev's detente as its theoretical framework for foreign policy. It has not returned to the fundamentally different view of irreconcilable hostility that was softened by Premier Nikita Khrushchev and even more by Brezhnev.
Also troubling is General Secretary Yuri Andropov's ill health and absence from public occasions since last August. This suggests a period of power struggle in Moscow. In such periods in the past, the Soviet Union has typically acted with caution abroad. In such periods, however, the Soviet military typically gained more power within the Kremlin. And these have been periods when potential leaders have not wanted to take domestic political risks in accommodating the West.
Monitor correspondent Gary Thatcher reports from Moscow:
Moscow has not yet said its final word on when - and if - the START talks will be resumed. Analysts here in Moscow are waiting for the Kremlin to make a definitive statement.
Some Kremlin-watchers expect that response to come from Soviet leader Yuri Andropov himself. He has not been seen at a public occasion for more than three months, but the Kremlin has continued to issue statements in his name. There are unconfirmed reports here that Mr. Andropov is almost fully recovered from an illness and is keeping limited office hours in the Kremlin.
The initial statement issued by Tass after the latest round of negotiations adjourned seemed to leave the door open for possible resumption. Tass said:
''A change in the overall strategic situation due to the beginning of the deployment of new American missiles in Europe compels the Soviet side to reexamine all the issues which are the subject of the discussion. . . .''
''The date for the resumption of the talks has not been set due to this,'' the statement concluded. It did not indicate how long such a reexamination might take.
Experts here would not be surprised if a tougher Kremlin response followed. Some recall that the initial Tass statement was relatively mild after the breakdown two weeks ago of negotiations on intermediate-range nuclear missiles. Later, a statement issued in Andropov's name detailed a series of Soviet countermeasures and declared that the Soviets would not return to the negotiating table unless the new NATO missiles were withdrawn.
The Soviets have played down the notion of combining the two sets of Geneva negotiations into one comprehensive discussion of both intermediate-range and strategic nuclear weapons. But some analysts think the Kremlin has not totally rejected the idea of combining the negotiations at some future date.