IF the Soviet Union is always eager to maintain its credentials as a superpower, as the Kremlin-watchers say, it's certainly not acting the part. The decision to try Mathias Rust, the 19-year-old West German who amazed the world by his flight into Red Square in May, would seem more typical of a banana republic than of a superpower eager to reach historic arms accords with the West.
Young Rust's unauthorized penetration of Soviet airspace did cause an almost immediate shake-up in the defense leadership in Moscow - a fortuitous opportunity, many thought, for Mikhail Gorbachev to install some of his own people in key positions. Meanwhile, the Soviets thanked the German pilot for pointing out the weaknesses in their air defenses; the young man was thought likely to be released and allowed to return home. The rest of the world sighed with relief that the Soviets hadn't decided to shoot first and ask questions later, as so tragically happened with the downing of Korean Air Lines Flight 007 in 1983.
But now Mr. Rust is to be tried on charges of illegal entry into the Soviet Union, violation of flight rules, and ``malicious hooliganism'' - though not espionage. He could be sentenced to up to 10 years in prison.
All this on the heels of West German President Richard von Weizs"acker's successful visit to Moscow, a Soviet decision to allow East German leader Erich Honecker to visit West Germany, and amid growing momentum for a US-Soviet arms accord. If this can happen during a ``thaw'' in East-West relations, what would happen if the Soviets got really angry? And they wonder why people have difficulty trusting them.
Clapping a teen-ager into prison for what turned out to be a harmless flight was unconscionable. Keeping him there is worse. He must be released at once.