Am I the only person who has seen the photographs published in all our newspapers? Americans are still furiously debating the nature of the Sandinista regime in Nicaragua and its intent in regard to the Arias peace plan, but surely that question has been settled conclusively by the photos that appeared on Nov. 3, the day after the opening of the Party Congress in Moscow. In them, Daniel Ortega of Nicaragua can be seen sitting next to Erich Honecker of East Germany and Gen. Wojciech Jaruzelski of Poland, in the section reserved for the leaders of Leninist governments in good standing.

Even under Mikhail Gorbachev, Soviet rituals continue to serve as very precise political indicators. Just as we would know at once that Gorbachev had been demoted if another Politburo member had been the first to speak in the recent Central Committee meeting, equally the seating of Ortega conveys a very definite message: the Sandinista regime has been admitted to the very exclusive club of governments that the Soviet Union regards as permanent, organic allies.

One might think that Moscow would be eager to recruit any and all to the ranks of its followers, so that Nicaragua's admission does not mean much, but that is not so. Even at his most pro-Soviet, Nasser was always kept at arm's length, in the friendly but non-Leninist category, and so have been numerous others.

There is a very good reason for the Soviet avoidance of the eager co-optation that is normal in politics. A most basic Soviet doctrine, which Gorbachev has taken good care to reaffirm at every opportunity, is that once a fully fledged Leninist regime is established, there can be no backsliding, no reversion to democratic governance, or any other form of government for that matter.

That assurance of the irreversible expansion of the faith obviously places a heavy burden on Moscow, which all Soviet leaders have always prudently tried to limit. In Eastern Europe the doctrine means that the Soviet Union must be ready to intervene, with force if necessary, to keep the local Communist Party in power. Even when invasion is successful, as in Hungary in 1956 and Czechoslovakia in 1968, the political costs are huge. And the mere readiness to intervene in Poland during the Solidarity years was tremendously damaging.

Beyond the immediate reach of the Soviet army, as in the case of Nicaragua, the irreversibility doctrine imposes even more demanding admission criteria. It requires first of all the difficult assessment that a faraway exotic regime in fact means to remain faithful to Moscow in all things, including military cooperation of course. Second, it demands an even chancier judgment that the regime has the determination and ability to remain in power. Third, it exposes the Soviet Union to risky out-of-area confrontations to protect the regime, once admitted, against local enemies, as in the case of Ethiopia -- or even against the United States, as in the Cuban case. Finally, it means that the Soviet Union will be faced by demands for military and economic aid. Grenada, for example, was refused admission on the basis of the last two criteria, and so was Nicaragua until recently.

While Americans continue to argue over the sincerity of the Sandinista acceptance of the Arias peace plan, it seems that in Moscow the question is regarded as settled. Leninist governments can make all sorts of tactical accommodations, but they must retain an unchallenged monopoly of power. If there were any suspicion that the Sandinistas might actually allow the democratization required by the Arias peace plan, creating the possibility of a peaceful change of government by free elections, Ortega would not have been seated where he was. For then an opposition victory in Nicaragua would be a Class A political defeat for the Soviet Union, and for Gorbachev. The writer is holder of the Arleigh A. Burke Chair in Strategy at the Center for Strategic and International Studies.