Brussels — The writer has previously been on assignment in Iran and writes on the area from his current base in Europe. The Soviet Union has suddenly become the ''No. 1 Satan'' in the Iranian lexicon, apparently taking over the role from the United States and Britain.
The Iranian government's expulsion this week of 18 Soviet diplomats and its banning of the country's communist party are only the latest examples of a deterioration in relations that has been going on for nearly a year.
Things began to go sour back in July 1982 when Moscow condemned Iran's first offensive within Iraqi territory. Despite official claims of neutrality, Moscow had previously backed Iran and had encouraged its proxies - Syria, Libya, and North Korea - to resupply the Iranian Army with ammunition and spare parts.
Toward the end of last year, relations between Moscow and Baghdad improved remarkably. And today the Soviets are said to regularly resupply the Iraqi Army.
The tension between Moscow and Tehran reached an initial peak last December. The Tass correspondent in Iran was expelled after failing to report on the bombing of the city of Dezful by the Iraqis using Soviet ground-to-ground missiles.
Soon after, in February, all the members of the central committee of the pro-Moscow group, the Tudeh Party, including secretary-general Nourredin Kianouri, were arrested in Tehran and charged with spying. Although an investigation had been under way for several months, sources close to the Iranian government report that the crackdown on the Tudeh was prompted by the confession of Vladimir Kuzichkin, a high-ranking diplomat from the Soviet Embassy in Tehran who defected to the West last fall.
The same Iranian sources contend that a transcript of Kuzichkin's debriefing by the British intelligence services was passed to the Iranian government.
Before his arrest, Mr. Kianouri was reported to have had regular contacts with the speaker of the Majlis, Hashemi Rafsanjani. Mr. Kianouri's family has strong ties with the Iranian clergy. Sources close to the Iranian government affirm that the ministers agreed on his arrest only after having seen concrete evidence of his involvement in spying activities. After nearly two months in jail Mr. Kianouri made a surprising television appearance last Saturday.
''Our members within the Iranian Army,'' confessed the leader of the Tudeh Party, ''helped us in the writing of reports that were directly sent to Moscow.''
''This was a betrayal of the revolution,'' added Kianouri, who was speaking in a clear and quiet voice and who seemed to be in full control of himself. The astounding performance lasted for 30 minutes.
Behind Mr. Kianouri was a board with a quotation of Ayatollah Khomeini: ''Great Britain is worse than United States. United States is worse than Great Britain. And the Soviet Union is worse than both together.'' This is a clear indication that the Soviet Union has now become the No. 1 Satan in Iranian rhetoric.
''We cannot believe it; he must have been tortured to say that,'' say Tudeh sympathizers in Western Europe.
In Tehran observers say Mr. Kianouri's public confession marked the end of the role played by the communist party in the Iranian revolution. A few hours later a new group of Tudeh militants was arrested and five other members of the central committee made public confessions. Although in his TV appearance Mr. Kianouri did not mention any names, Iranian diplomats in Europe assert he gave to the government a list of the Soviet diplomats in contact with the Tudeh.
Western diplomats in Tehran say that the Soviets had anticipated the Iranian decision ''and were ready to leave'' when they got the news of their expulsion. Among those expelled is the Soviet consul in Isfahan who is accused of having stirred up troubles among the workers of a huge steel mill plant managed with the help of Soviet experts.
During recent weeks, Radio Moscow in its Persian programs has relentlessly criticized the attitude of the Iranian government toward the Tudeh Party, explaining that it was the result of the growing influence of ultraconservative religious elements within the government and Parliament.
Observers in Tehran report that although it is true that tensions are growing between supporters of a capitalist-type economy and supporters of reforms in Iranian society, there is no sign of divergences in the attitudes toward the communist party and the Soviet Union.
Mr. Kianouri and his friends will probably be tried very soon. The Iranian government might be tempted to organize a big show and broadcast the trial live on TV and radio.