Having earlier portrayed the pending Geneva talks as an achievement for its diplomacy and a harbinger of better East-West times, the Reagan administration now offers the talks as a useful but necessarily modest occasion that will merely let Secretary of State Shultz and Foreign Minister Gromyko start trying to get together on arms control. The scaling down of public expectations is wise. The whole grim record of arms control failures and frustrations is being brought to Geneva, along with a lot of mutual distrust. Just to agree on a workable agenda will be a strenuous and time-consuming task.

What is reasonable to ask of the two sides? That each at least address the other's principal anxieties. On the American side concern focuses -- as it must -- on the question of Soviet compliance with past accords and on the threat that Moscow's heavy land-based missiles continue to pose to American land-based missiles, the American command and control system and American peace of mind. On the Soviet side, concern centers on the United States' quickening deployment of numerous accurate new offensive weapons and on the possible eventual success of President Reagan's missile-defense project: taken together the ingredients of a capability for a first strike.

It is not necessary -- nothing in the history of arms control indicates it is feasible -- to tackle the totality of the two sets of concerns. The test will be whether appropriate pieces can be isolated for the purpose of renewing a negotiation. Mr. Reagan has high arms control ambitions. Still, ambition can be a trap. The Soviet-American circuit is frail and easily overloaded. After much roughness and a long hiatus, what is most needed is a sure start.

Too little is known of the debates that produced the position Mr. Gromyko is carrying to Geneva. Too much may beknown of the debates that produced the American position: so much that perhaps insufficient attention is being paid to the effect that the very process of a negotiation -- if one actually begins -- can have on that position.

The Russians assail Mr. Reagan for launching "Star Wars." But of course. They would love to see him abandon, for no price at all, a program that may greatly widen the American technological edge even if it never gets beyond the current protracted research stage, which it may not.

The American critics of "Star Wars," forgetting perhaps that Mr. Reagan is a newly and strongly reelected president with a determined grip on the idea, assail him for saying he is unwilling to trade it away. But in a certain context, one which Moscow can shape by addressing American fears, the unnegotiable surely becomes negotiable.

It is uncertain whether the representatives of the two countries in Geneva will be able to break past the familiar obstacles. But it is good to see Mr. Gromyko and Mr. Shultz heading there. Dialogue is important. It offers the essential reassurance that the two superpowers, with their immense responsibility to the whole globe, are seeking a greater measure of control over the common destiny.