Sometimes, in the course of negotiations, one side is offered most or all of what it wanted initially. Yet the offer no longer looks attractive. Attitudes have hardened. What would earlier have been considered -- and what outsiders would still consider -- a major victory can be overlooked in the process.

A few weeks ago, the Soviet Union offered to bring the total of its long-range missiles and bombers down to 1,800. This is about a 25 percent cut from its present total. The cut does not go as far as the U.S. proposals, but it goes far beyond what the Soviet Union has ever offered. The resulting arms level would be close to the level that President Carter proposed in 1977 and close also to the overall levels that Sen. Henry Jackson proposed in 1974 and 1975. By these standards, achieving such a level should be considered a major accomplishment and a step forward in arms control.

The Soviet proposal comes with strings attached. Major Soviet steps in the direction of U.S. positions are unlikely to come without strings attached. If experience is any guide, the strings can be dealt with successfully. In particular, we usually do quite well with regard to systems that we have under development and that are essential to the effectiveness and stability of our forces, such as cruise missiles. The negotiations about intermediate-range weapons in Europe that are going on simultaneously can be helpful: they provide an opportunity to deal with systems that one side or the other does not want to deal with in START.

The Soviet proposal is couched in the language of SALT II, with long-range missiles and bombers counted together under a common limit, rather than in the START language, which deals with missiles and their warheads. A compromise should be possible: there were missile sublimits agreed to in SALT II, and these could be negotiated further down. The SALT II common limit would allow both sides to shift forces from less survivable modes to more survivable ones, a provision that will become particularly important as the total of forces goes down.

The Soviet offer comes at a time of transition within the Soviet Union. There are some indicators (ambiguous, as such indicators always are) that the priorities of the past decade could shift in the direction of lessening the burden of military expenditures in the Soviet Union. The overtures to China are one such indicator. The offer of more significant strategic reductions than the Soviet Union has ever offered before is another.

None of this means that the Soviet Union is ready to change its internal and external behavior to suit us better. Nothing save the passage of time can bring about fundamental changes in societies. Change, when it comes to the Soviet Union, will probably come in directions that neither side is predicting today. But the Soviet Union (or at least some group there) may be ready to bring the strategic confrontation to lower and less dangerous levels.

The Reagan administration is trying to nail down Soviet intentions, speed up the pace of reductions and negotiate away one-sided provisions. Doing these things is right and important. But it is also right and important to note that what has happened is quite significant by all recent standards. That it happened must be counted a plus for the administration and its approach. It could also provide a basis for a more stable nuclear balance in the next few years.