THE announcement of an intermediate nuclear force (INF) agreement between Washington and Moscow is welcome. It was long in coming - a year ago such a pact was thought to be at hand. It is modest, affecting 4 percent of the superpowers' nuclear arsenals. But it does, as the Reagan administration maintains, represent the first agreement to reduce the numbers of nuclear weapons rather than limit their buildup.
It represents, too, a cresting of the nuclear arms race - it does, that is, if President Reagan's Strategic Defense Initiative, or ``star wars,'' program is not perceived as an added dimension of the strategic weapons equation. The same goes for the administration's loose interpretation of the antiballistic treaty. This interpretation is stiffly opposed by congressional stalwarts like Sen. Sam Nunn. It could rouse enough opposition to jeopardize ratification of the INF pact that Mr. Reagan and Mikhail Gorbachev want.
This is very much a superpowers agreement. On the Soviet side, its impulsion lies in the economic realities and domestic objectives Mr. Gorbachev faces. Lower East-West tension makes it easier for Gorbachev to direct national resources to the domestic economy, pursue economic reform, and stave off critics.
On the United States side, the pact is impelled by Reagan's desire to take the personal and historical profits from his long investment in militarily strengthening the US and standing up to the Kremlin. It reflects too a recognition of the popularity within the US of a more reasoned relationship with the Soviet Union.
It raises concern in Western Europe, where some see any move toward denuclearization as leaving the NATO allies more vulnerable to the East bloc's superior conventional forces. Obviously, however, any arms agreement must meet the superpowers' own objectives. And allies benefit from eased tensions.
At home, the dismay Reagan faces among longtime conservative allies could be fueled by too fast an INF phase-out. And opposition on the ABM matter and along star-wars lines could feed into the ratification battle.
Still, the prospect of a Gorbachev visit to the US later this year, and of a Reagan visit to the Soviet Union next year, perhaps to initial a nuclear test treaty, represents a positive turn in US-Soviet relations - a turn that should not be impeded.