Testing time for US-Soviet relations. Will superpowers link key issues, or pursue them one by one?

``Thaw'' was the word for Soviet-American bargaining in the late 1950s and early 1960s. ``D'etente'' was the label for the Metternich-style deal-balancing of Kissinger, Nixon, and Brezhnev in the early 1970s. What should the world call the superpower warming trend now starting up? And what does it mean outside the missile sphere?

Do the globe-ranging talks between the Soviet and American foreign ministers in Washington and New York mean, for instance, limited cooperation on the Iran-Iraq war? On Nicaragua? On getting Vietnam out of Cambodia? On Israeli-Jordanian-Palestinian talks? On Afghanistan? On trade? On human rights?

The name for the new warming cycle is not terribly important. (President Reagan would probably prefer that it not be ``D'etente II'' because he believes in symbolism and thinks Moscow took advantage of D'etente I.)

Much more interesting is the question of whether United States Secretary of State George Shultz and Soviet Foreign Minister Eduard Shevardnadze and their bosses will adopt the ``compartmentalization'' style of the Lyndon Johnson era or the ``linkage'' approach of Henry Kissinger.

That's yawn-producing, striped-pants jargon. But it is important. During the Vietnam war period, Washington wanted to keep its relations with Moscow on many subjects relatively calm while it escalated the war against Moscow's client - bombing around Soviet ships supplying weapons through Haiphong harbor. Hence ``compartmentalization.''

During the '70s, Mr. Kissinger and his successors sought to ``link'' progress on arms control and trade to progress on other collision points. Congress linked East-West trade to emigration of dissidents from the Soviet Union. The US and Europe formally linked human rights to recognition of the Soviet sphere of influence in Eastern Europe.

Standard diplomatic wisdom at the moment holds that Mr. Shultz has not linked the Gulf war or Central American struggle to East-West missiles and East-West trade, even though he and the affable Mr. Shevardnadze have spent many hours talking about world trouble spots on which they differ.

They may not be linking them to arms control in a package deal. But they are obviously trying to find ways in which they can simplify their respective problems by limited cooperation or balancing concessions. Soviet leader Mikhail Gorbachev is strongly interested in cost effectiveness. Shultz is trying to point out conflict areas in which a retraction of Soviet power would not lead to an advance in American power.

All these areas involve risk. None more so than the Gulf war, which this week veered away from discussion of a cease-fire and outside mediation to naval collision and Iranian denunciation of the UN as peacemaker.

Events in the Gulf resembled, in fact, a high-tech version of the Boxer War at the opening year of this century. In that encounter, Chinese extremists took on multinational forces of Western Europe and Russia. The Boxers were a clique of zealots who split with pragmatists in the Chinese court. They threatened Western embassies, as the Iranian Revolutionary Guards have. Some of their troops were taught to believe they were impervious to the Western devils' bullets; some Iranian irregulars reportedly believe the same about the Great Satan's weapons. The Europeans were sometimes united, sometimes divided, as they were through much of the 19th century in their struggle to open Chinese trading ports.

There are two immediate focuses in the rekindled Iran-Iraq war. One centers on high-profile US (and quieter European) naval moves to keep the oil tankers moving out of the Gulf ports to the world.

The other focus is on the UN. There Shultz and Shevardnadze will be talking confidentially with dozens of foreign ministers, trying to sell their rival ideas on world order. They will also be discussing with the other powers of the UN Security Council what to do about Iran's refusal of that Council's demand for a cease-fire and withdrawal to old boundaries.

It is ironic that the Reagan administration, some of whose officials have scorned the UN (even suggesting it be allowed to sink), is suddenly demanding that that organization's most vigorous, and least used, power be invoked. Washington is asking that the UN Charter provision for mandatory sanctions to halt a war be activated.

Neither Moscow nor Peking, among the Council's veto-wielding powers, is believed ready to go along. After months of quiet bargaining, both had agreed to the unanimous UN resolution asking a cease-fire in the war. But instead of a trade and weapons boycott against Iran, it seems likely that Moscow might at most decide to help in behind-the-scenes bargaining with Tehran to return to talking about terms for accepting a cease-fire.

In the longer range, expansion of the US-Soviet thaw may bode well for the UN and its peacekeeping and peacemaking authority. When its two superpower members are icy, the world organization is effectually frozen. When their relations warm, efforts to deal with regional disputes come out of mothballs.

But much of the momentum on major trouble spots must come from the Big Two themselves. The cost-conscious Mr. Gorbachev has long telegraphed that he is not interested in endlessly subsidizing Nicaraguan Marxists or quasi-Marxists. Some specialists believe he is pressing Hanoi to put in place a pro-Vietnam government in Cambodia and withdraw as promised in 1990. (But Moscow has continued to expand its use of US-built Cam Ranh Bay naval facilities in Vietnam. That base, only some 700 sea miles from the Marxist-rebel-troubled Philippines, creates a long-range but potentially worrisome leverage over Japan's main fuel lane.)

Shevardnadze will also be seeing Israeli Foreign Minister Shimon Peres in New York. Despite entrenched differences, their two nations are moderating their positions toward each other. At their last meeting, Mr. Peres reportedly quoted the great Georgian epic poet Rustaveli to the Georgian Shevardnadze.

The Soviet minister is trying to persuade the Israelis that Moscow should play as large a role in the Mideast as Washington. The Israeli minister is trying to persuade his counterpart in the Soviet Union that that might happen, if Moscow shifted its position to more even-handedness between Israel and its Arab neighbors.

Even-handedness appears to be a Kremlin goal in the Iran-Iraq struggle. It will be interesting to see if there is any shift in Moscow's approach to the struggle at the other side of the Mideast.