The Euromissile arms control talks in Geneva are entering a critical phase. Washington and Moscow must now decide whether they really want arms limitation - or whether they prefer an open-ended arms race. Neither the Reagan administration nor the Kremlin has yet made this fundamental decision.
This is the picture that emerges from conversations with a representative sample of American and Western European diplomats and officials.
For both superpowers, several events prod them toward a decision: US Vice-President George Bush's European tour Jan. 31 to Feb. 10; the March 6 West German election; the December deadline for stationing new NATO missiles in West Germany, Britain, and Italy; and the 1984 American presidential election.
If mutual Euromissile ceilings are not agreed on before the NATO deployment program develops its own momentum - and before the American presidential election makes a political football out of arms control - then a potentially dangerous new spurt in the arms race and world tensions seems unavoidable.
In both East and West, however, the weight of inertia favors inaction until it is too late.
In Moscow the greatest incentive to Euromissile arms control is the desire to block deployment of America's 1,200-mile range, under 15-minute flight time, 130 -foot-accurate Pershing II missile. The Pershing II - which is to begin deployment in West Germany this December - is a hard-target killer that (in conjunction with US strategic weapons) could constitute a ''first-strike'' weapon against major Soviet missile sites or command posts.
The 108 Pershing IIs scheduled for West German deployment would not by themselves threaten a surprise attack. But their short flight time - and their potential for reload as well as possible future stationing in Turkey or elsewhere on the Soviet periphery - clearly worries the Soviet Union.
The Soviet priority on blocking the Pershing IIs (as distinct from NATO's planned 1,500-mile range, almost two-hour flight time, 260-foot-accurate cruises) was suggested in the ''exploratory package'' worked out by chief Geneva negotiators Paul Nitze and Yuli Kvitsinsky last July. It was subsequently rejected by both home governments.
In that probe Mr. Kvitsinsky suggested an equal 75-75 balance of Soviet and American land-based Euromissiles if the American Euromissiles would consist only of cruises, with no Pershing IIs. Notably, to achieve this renunciation of the Pershing II, Kvitsinsky was tentatively ready to forgo the Soviet claim to compensation for France's independent and Britain's semi-independent nuclear missiles, and to leave sea-based missiles out of the balance.
He was also tentatively ready to scrap some 150 of the Soviet Union's already-deployed 3,100-mile range, under-15-minute flight, 1,000-foot accurate SS-20s; to drop, effectively, the complex issue of the nuclear aircraft balance; and to freeze already deployed SS-20s in the Far East at 90.
Western opinions are mixed as to how much authority Kvitsinsky actually had to make this probe - and why the Kremlin rejected it. Some veterans of East-West negotiations argue that Kvitsinsky could never have made such a bold bid without permission from above. Other veterans argue that in fact the critical breakthroughs in East-West talks have always come through the negotiators' exceeding their instructions and then securing their governments' approval.
In any case, Moscow did distance itself from the Kvitsinsky probe, and did so while the late President Leonid Brezhnev was still living.
What the Nitze-Kvitsinsky package did suggest, was (1) that Moscow thought it could not rely on the European antinuclear movement to block new NATO deployments and would itself have to negotiate hard to reduce those deployments; and (2) that Soviet priority lies in preventing Pershing II deployments.
Even if these assumptions are true, however, a major question arises in this Kremlin succession period about whether General Secretary Yuri Andropov would in fact have the domestic authority to make the concessions required for any arms control agreement.
The insecurity and fluidity of a succession period, with its pressures for consolidation and loss-cutting, do provide an incentive for arms control. The fear that a chauvinist rival might outbid the incumbent leader by accusing him of softness toward the West, however, increases resistance to arms control.
How these factors will eventually balance out in Kremlin thinking is not yet clear to Western observers. For the moment Moscow is simply performing a holding operation. It is dribbling out hints of concessions in calculating a European balance of warheads rather than missiles, and possible destruction (rather than only relocation) of any SS-20s withdrawn from the European theater.
The Kremlin is also encouraging European antinuclear movements and the West German Social Democrats to campaign for nuclear-free zones, to count French and British missiles against Soviet missiles, and to press for unilateral postponement of new NATO missile deployments.
At the same time, however, Moscow is conspicuously keeping its lines open to the conservative government in Bonn that is generally expected to be reelected in the Mar. 6 election - and that is committed to NATO deployments in December. If the Kremlin is serious about mutually agreed arms control, then, it could quickly renew the Nitze-Kvitsinsky search for a real compromise if and when the government of Helmut Kohl gains a new four-year mandate.
Halfway around the world, in Washington, the arms control climate is no clearer than it is in Moscow.
In a curious way the January dismissal of Arms Control and Disarmament Agency director Eugene Rostow in part for supporting the Nitze-Kvitsinsky probe has cleared the air. The hard-line ideologues who forced Rostow's resignation are unlikely to attack Nitze, however.
Yet the battle for President Reagan's ear on arms control has not yet begun in earnest. And the Defense Department remains determined to bar any arms control agreement other than total dismantling by the Soviet Union of all of its already deployed SS-20s in return for no future NATO deployments of new American missiles.
This ''zero option'' (the official US proposal in the arms control talks) is by now generally regarded in Europe as a good opening that will, however, soon have outlived its usefulness and should be modified - unless the West wants to ensure arms control failure.
Given the strong antinuclear movements in two of the NATO countries scheduled for December deployments - West Germany and Britain - the conservative governments there have discreetly let Washington know that they do not in fact want the West to sabotage arms control. Instead, they would like to see movement toward a more flexible Western position after the West German election.
These conservative governments can get reelected while publicly supporting the zero option, the West German experience suggests. But they cannot guarantee social peace once deployment starts if Washington is seen as the cause of arms control failure.
This European interest in more US flexibility has been extremely discreet, however. And it is unclear to observers in Europe if the message has gotten across to the Reagan administration. The hope here is that Bush will convey the message to Reagan after his two weeks of European consultations and public relations.
Nor is it clear just who might be willing to initiate a campaign for greater willingness to compromise in Washington. As nearly as can be learned at this distance, all those who would like to see greater US flexibility seem to be hoping that someone else will actually step forward and confront Reagan with this idea (and take the accompanying heat from the Defense Department).
All this leaves an arms control limbo in Washington at the moment that parallels the limbo in Moscow.