IN theory there is an easy solution to the problems of Afghanistan and Nicaragua. Indeed, Mikhail Gorbachev is reported to have proposed it to President Reagan during the Washington ``summit.'' The Soviet leader is said to have suggested that he would stop helping the Sandinistas if Ronald Reagan would stop supplying weapons to the Afghan resistance. According to the reports, neither officially confirmed nor denied, Mr. Reagan either did not hear the suggestion or managed not to hear it.
If the non-noticing of the idea was deliberate, there is a reason. The two situations are not ``symmetrical.'' That word has been used of late in connection with talks over arms cuts that lie ahead. Lack of symmetry is going to be a major problem. The Soviets are well ahead of the NATO allies in conventional land power in Europe, but the allies are far ahead in conventional sea power and long-range air power.
Matching these asymmetrics to produce a fair balance is going to be a nightmare for the experts. Getting a ``deal'' linking Afghanistan with Nicaragua is probably impossible. The situations differ in one key respect and many lesser respects.
The big difference is that if the Soviet Union were actually to pull its troops out of Afghanistan, the Soviet puppet President there would be wise to take the last helicopter out. His chances of survival after that would be negligible. The Soviets would love to leave behind a government of their choosing that could stay in power after they leave. But that is pie-in-the-sky stuff. The bitterness most Afghans feel against the Najibullah regime is enormous.
The Afghan resistance movement has many factions. Forming a new government if and when they take over will be difficult. But they all agree that Dr. Najibullah must go. They are now maneuvering over whether to allow one or two lesser members of the puppet regime to remain at least briefly in some ``transition'' government. Apparently some would accept that as part of the price of getting the Soviets out; they know they could soon take care of the ``holdovers,'' if that seemed desirable.
But the Soviets know, as does everyone else, that Najibullah and a puppet regime taking orders from Moscow are as much a thing of the past when the Soviets leave as the pro-US regime in Vietnam was after the last Americans left Saigon.
Nicaragua is a different matter.
Daniel Ortega Saavedra was not imposed on the Nicaraguan people by Soviet bayonets. There are a few Soviet advisers in Nicaragua, but no fighting troops. Mr. Ortega is the product of a bona fide revolution and an election that foreign observers at the time said was honest, despite an opposition boycott.
Ortega is not a puppet of the Soviets. He is a native Nicaraguan. He has legal, if flawed, title to his position, and many loyal supporters. In a free election today he might win a majority of the vote - although that cannot be proved either way.
The Soviets have never been generous with him. They keep him on short rations for oil and have never given him the fighter aircraft he has repeatedly requested. Western Europe has helped.
Thus if the Soviets cut off Ortega entirely he would not topple, as would Najibullah in Afghanistan. He might last a long time on his own resources.
Thus President Reagan is not going to overthrow Ortega and the Sandinistas by arranging a cutoff of Soviet aid to them. To overthrow Ortega, he will have to get much more aid from Congress for his contras than Congress seems ready to offer - unless Ortega again does some very stupid things.
It would be ironic if Reagan succeeded in getting the Soviets out of Afghanistan but can't get Ortega out of Nicaragua.