THE ESPIONAGE cases against Marine guards at the U.S. Embassy in Moscow have begun to fall apart. There are two possible explanations. Either the original charges were blown out of proportion and no serious offenses occurred, or a serious case was mishandled by investigators and now a prosecution is impossible. In either case there appears to have been a terrible bungle, and an explanation is owed the public.

The cases came to light in January when the Marine Corps revealed that Sgt. Clayton Lonetree had confessed to giving secrets to a woman who was a KGB agent. Sgt. Lonetree was then said to have implicated Cpl. Arnold Bracy, who was arrested in March on suspicion of espionage. The two were later charged with having allowed Soviet agents to enter and roam through the embassy, where they were alleged to have had access to classified documents and codes. Subsequently, two other Marines who had served in the Soviet Union were also charged. Staff Sgt. Robert Stufflebeam was said to have socialized with Russian women; Sgt. John Weirick was held for questioning in connection with espionage charges.

All 28 guards at the Moscow Embassy were ordered home in April. This was, of course, a heck of a story involving, as it did, international intrigue, spies, sex and an elite corps of young military men. Cabinet members and other high government officials expressed outrage and hinted of irreparable damage to the security of the Western world. Members of Congress denounced the American ambassador and the Moscow embassy staff for a horrendous failure to maintain security, and called for their punishment.

Then gradually, the story began to unravel. The most serious charges against Sgt. Lonetree -- that he had allowed Soviets inside the embassy -- have been dropped. Those stemming from his own confession that he had been involved with a woman are still pending. Sgt. Weirick was never charged and has been released. Cpl. Bracy recanted his confession, saying it had been coerced. In the absence of any physical evidence at all to substantiate the confession, the Marines have now dropped the charges against Cpl. Bracy and released him from custody.

What's going on? Is there a major scandal here or not? Are we left with two cases of young Marines who in the end will be charged only with having Russian girlfriends? Was there or wasn't there a real breach of security at the embassy? Were KGB agents inside the building at all? And if all of this did happen but no prosecutions can be brought, what happened during the investigation that sabotaged the cases? The slow sorting out, the trickling away of charges won't do. The Pentagon must explain what has happened and why. The public, the young men involved and the Marines Corps deserve no less