WARSAW -- What pushed Mikhail Gorbachev to stage the public humiliation and dismissal of his protege in perestroika, Boris Yeltsin, from his post as leader of the Communist Party apparatus that runs the city of Moscow?

"We should not hurry past the obvious," a midlevel Polish official said in hushed tones in a threadbare office shortly after Tass had carried the initial accounts of Yeltsin's demeaning downfall. "The Moscow party apparatus has a lot to do with public security in the city. If you are sitting in the Kremlin at a time of great change, you do not want to have the smallest doubts about the stability of the person running the apparatus."

Gorbachev's sacrifice of the man he put in the Moscow job to push perestroika, or restructuring, will be debated by Kremlinologists for months to come. It is a particularly traumatic event for the members of the East European political elites who have allied themselves with Gorbachev's efforts to revitalize Soviet society. Primarily in Poland and in Hungary, these elites bring their own heavy preoccupations to the arguments about Gorbachev's authority and intentions at home.

In discussions in Budapest and Warsaw this month, a strikingly cohesive view emerged of a series of tactical retreats forced on Gorbachev after concern began to mount in the Politburo about public security in Moscow this summer. East Europeans I spoke to put less emphasis on the idea of a winner-take-all battle over reform, although elements of this were also involved.

The view that security was the dominant issue would explain the rapid reversal of fortunes that occurred in Moscow over the past six months. Gorbachev emerged from the June plenum of the Central Committee triumphant, able to place his allies on the Politburo and in charge of the armed forces. Senior officials in Washington judged that he had finally turned the corner.

When a group of Crimean Tatars staged demonstrations in front of the Kremlin in July and did not get their skulls bashed in, it seemed to promise a beginning of some form of glasnost, or openness, in the Soviet capital. Watching from a hotel balcony on the final evening of the protests, I was stunned to see Tatar demonstrators march out of Red Square shouting slogans under the protection of Soviet traffic policemen. Soviet citizens on the sidewalk were openly amazed and, more to the point, openly angry over this un-Moscowlike spectacle.

In retrospect, the demonstrations marked the high point of Gorbachev's authority and of the glasnost movement for this year at least. It was not long afterward that Gorbachev disappeared on his 56-day vacation, and the head of the KGB, Victor Chebrikov, openly attacked glasnost in a speech in Moscow.

The KGB's attitude toward glasnost had been something of a mystery until Chebrikov's speech. Syrupy praise for Gorbachev from some Soviets who were authorized to have contacts with foreign newsmen and who were thought to have ties to the security agency had suggested there was support for glasnost somewhere within the organization.

But Yeltsin appears to have been an early and particularly important target of those who were out to discredit reform in ways that carry the mark of a proficient and confident security operation. In a remarkable disclosure, the Paris daily Le Monde published last year the transcript of a tape made at a meeting of Moscow party leaders in which Yeltsin sounded at times like a Robespierre in waiting. Another tape recording of Yeltsin addressing a group was turned over this year to The New York Times.

It may have been no accident, then, that when the conservatives took the floor at the Moscow city party meeting last week to denounce Yeltsin, one of their heaviest charges was that he had talked carelessly about party matters around foreigners.

In disgrace, Yeltsin has achieved a stature in Soviet political life that his successes never brought him. His fitful stabs at reform have taken on a tragic quality and he has become a symbol of a future that may never have been in Gorbachev's mind in the first place. A representative view in Eastern Europe seems to be that once the Washington summit is out of the way, Gorbachev will try again on restructuring from what he hopes will be a steadier base in Moscow.