Tension, pressure and sometimes confusion are escalating in capitals throughout Western Europe as NATO nears a decision on whether to modernize its arsenal of atomic weapons in Europe -- an issue crucial to the future cohesion of the alliance and possibly even to the future of U.S.-Soviet strategic arms talks.

Officials here, in Belgium and the Netherlands say continued Soviet verbal threats to Western Europe, such as the one voiced Thursday by Defense Minister Dimitri Ustinov, not to allow stationing of new U.S. Pershing 2 and Tomahawk cruise missiles on their soil actually could backfire on the Kremlin. It could increase Western resolve to counter new Soviet arms with a weapons modernization plan.

On the other hand, the Soviet tactics, which include undefined offers to negotiate reductions in such weapons, may wind up splitting alliance unity at a critical time, with the resulting ill will possibly hurting the chances for ratification of the U.s.-sOviet strategic arms treaty now before the Senate and the prospects for the followup SALT III negotiations.

The stakes, therefore, are high.

For NATO, the next two months may determine if it will continue to be able to come to grips with tough decisions.

For the West European allies, what is at stake is their ability to carry out one of the great juggling acts of recent years: to beef up their defenses in the face of an already existing Soviet military advantage without damaging their policy of East-West detente in Europe and good relations with Moscow.

The complexity of the issue, Western diplomats concede, is staggering. The political maneuvering is skillful. And, while the consensus among ALLIED OFFICIALS IS THAT IN THE FINAL SHOWDOWN SALT II will be ratified in Washington and a combined NATO arms modernization and arms control plan will be approved in Brussels in December, the outcome is still far from certain.

The NATO decision involves a U.S. declaration of readiness to begin production on new medium-range missiles and to begin stationing about 572 of them late in 1983 at U.S. bases in Britain, West Germany, the Netherlands, Belgium and Italy. The idea is to balance the new Soviet long-range, mobile and accurate SS20 missiles, about 100 of which are already deployed, and new Backfire bombers.

The United States has maintained 7,000 tactical atomic weapons in Europe. But the vast majority are short-range battleield artillery weapons and bombs. The significance -- and the sensitivity -- in Europe to the new weapons is that they would have the range to strike targets inside the western Soviet Union and balance the new Soviet ability to strike behind the lines at NATO arsenals.

Linked with the NATO arms plans is a second proposal crucial to Western Europe's desire to maintain detente with Moscow. That is a Western offer to begin negotiations quickly with Moscow on these weapons so that perhaps reductions on both sides can be achieved during the years between production and stationing.

The arms control plan is perhaps even more at the center of the current international tension because it is widely held that the plan could only be pursued in the next round of SALT -- that is, following SALT II ratification.

The pressures escalated further when Soviet Prsident Leonid Brezhnev, in a major speech in East Berlin Oct. 6, announced some unilateral Soviet troop and tank pullouts from East Germany and offered to negotiate on medium-range missiles. But Brezhnev said he would talk only if the West dropped plans to station the new missiles in Europe. He has not disclosed if he is interested in talking about the Soviets' missiles or just some 500 older and less capable ones.

The Brezhnev speech, however, has had considerable impact, especially among left-wing Europeans, and added to confusion in the West.

President Carter, his national security adviser Zbigniew Brzezinski and Defense Secretary Harold Brown have argued that the West Europeans are insisting that they could not go along with the NATO arms modernization program unless SALT II is ratified because otherwise there would be no SALT III and thus no forum in which to pursue the interlocked NATO arms control proposal.

There is no doubt that all of Western Europe's leaders desperately want the Senate to approve SALT II. If it fails, they fear a new cold war instead of detente, and chaos in efforts to get public opinion to back government action on arms issues.

But in fact no West European leader has stated flatly without SALT II ratification, they will reject the missile modernization plan.

Indeed, there is evidence that some Europeans are distancing themselves a bit more from that position so as not to tie their hands if SALT II fails and so as not to appear to be pressuring the Senate unduly. Sen. Sam Nunn (D-Ga.) warned a group of European specialists last week that failure of SALT could make it even more important that Europe improve its defenses.

President Carter said in his Oct. 9 new conference that (West German) "Chancellor Helmut Schmidt said, I believe yesterday or the day before, that a prerequisite to a decision by our NATO allies to take these steps, which he considers vital for the security of NATO, is the passage of Salt II."

The White House, queried about this, says it refers to a news account of a Schmidt speech in Nuremberg. Actually, however, in that speech Schmidt said only that he "hoped" very soon for SALT III negotiations over European system and that "a precondition for that is ratification of SALT II," which is well known. Schmidt did not mention this in connection with the NATO modernization plan.

A few days earlier, West German Defense Minister Hans Apel was in Washington and Brzezinski ushered Apel before some television cameras. This apparently surprised Apel, who thought the cameras has to do with the pope's visit.

Brzezinski, according to German press reports confirmed by officials here said, "Hans, why don't you explain the connection between modernization of medium-range weapons and SALT II?" Apel did, and before he left Washington, government spokesman Klaus Boelling in Bonn corrected any impression that there was a firm link between the two in Bonn's stance.

In subsequent interviews, numerous Bonn officials and Boelling have reaffirmed that, as Boelling says, "of course, we are interested in ratification and modernization, but there is no direct link."

Belgian Foreign Minister Herri Simonet also has tried to draw Washington's attention to the importance of SALT ratification in Europe but also, as one of his top aides said last week, "without any direct or automatic link." The problem, he said, "is that it would be more difficult to get arms modernization through public opinion if SALT was not ratified" because there would be great confusion about arms races and arms control.

In the Netherlands, most resistant to the New arms, Defense Minister Willem Scholten said this week, according to chief spokesman A. J. Sligting, "that the government will not unconditionally commit itself to the tactical nuclear arms modernization as long as the fate of SALT II is unknown." However, the spokesman said, "that is not the same as saying flatly -- no SALT, no nothing."

Two of the smallest countries, Holland and Belgium, are at center stage. Dutch opinion, which carries greatweight in Western Europe, includes a large antinuclear constituency.

"There is not a soul in Holland who could tell you today how the country will go on the NATO plan," a top official said. Nobody disagrees with him. Some diplomats suggest that the prospect of isolation within the alliance could ease the Dutch reluctantly into the fold.

The ruling Dutch coalition is conservative. But it has a tiny majority and a bloc of sure antinuclear voters, so the government could collapse if the issue comes to a vote of confidence.

Although officals in neighboring Belgium indicate their government will probably go along with the NATO plan, they say a collapse in the Netherlands, or even a strong rejection, could influence Brussels.

The challenge for NATO has been to find other nonnuclear countries to share the stationing of the weapons with West Germany, which refuses to be alone on such a crucial issue certain to irritate Moscow.

Britain's stance is clear in favor of the dual-approach NATO plan. But because Britain is a nuclear power, it is not perceived here as a qualifying partner with Bonn in accepting the missiles. West Germany does not want to be seen as somehow achieving nuclear status.

Aside from saying Bonn does not want to be the only nonnuclear country stationing these weapons, which implies that only one more is needed, Schmidt has also talked publicly of "many" countries sharing the burden.

Top Bonn officials say West Germany has never linked its approval of the NATO plan to approval by any other specific country, as Holland. On the other hand, the Germans keep looking for company.

Italian officials were quoted last week as being "reserved and cautious" about the NATO plan. They are, however, generally viewed as willing to go along -- although only Italian participation might not be enough for Bonn.

There are two other possibly decisive tactics at work in the Netherlands.

One is prospect that there will be no conclusive parliamentary debate or vote before the December NATO meeting, which some believe might then enhance chances of approval because the government presumably would already have given its okay in Brussels and the isolation factor of a subsequent no vote by the Dutch might be even more obvious.

The other is that a proposal to remove large numbers of old U.S. atomic artillery shells and bombs from the Netherlands in return for stationing a smaller number of more modern weapons -- better at deterring attack -- may help soften the opposition. Dutch officials say the number of proposed new NATO weapons is too high.

In a week or so, Dutch former prime minister Joop M. den Uyl, leader of the opposition Socialist Party, is to visit Bonn to discuss the tense situation with Schmidt.

On Nov. 21, it has been announced, Soviet Foreign Minister Andrei Gromyko will also visit Schmidt, undoubtedly to press the Soviet offers and threats once again as the crossborder operations in Europe continue until the deadline.