On Tuesday Cuba commemorated the 31st anniversary of the defeat of the tyrant Fulgencio Batista and Fidel Castro's triumphant entry into Havana.

More to the immediate point, 10 million Cubans will be preparing themselves for a calamitous change for the worse in their daily lives.Cuba is isolated, hard-pressed by its former friends, confronted by the United States, short of money and unable to borrow more. Since there's little or no cash for food imports, city dwellers are being sent to the countryside to help grow their own food in case there isn't enough to eat.

Castro warned before Christmas that depending on the situation in the Soviet Union and the Persian Gulf, there might be no gasoline at all in 1991. The days of guaranteed and subsidized Soviet oil are long passed as Moscow fights its own battles for economic survival.

The "zero fuel option" has already led to the replacement of tractors with 200,000 oxen in the countryside. Two hundred thousand bicycles have been imported. Next year 300,000 more will arrive. A bicycle factory is being built.

"Many people are worried about exercising when they get home. Now you can do it on the way," Castro said.

Even more ominously, Cuban television announced two weeks ago that nine people had been convicted of the crime of being professional line-standers. Those who made money by standing in line for others would receive no mercy, Tele Rebelde warned, indicating that shortages of consumer goods of all kinds would continue.

The political system Castro built up over three decades is crumbling. The first and most important pillar of the system is the deep Cuban nationalism, born of decades of bloody fighting against colonial Spain in the last century and a scarcely less bitter struggle against the United States in this century.

The second pillar has been the convenient ideological framework of "Leninism." Castro freely adopted this ideology which allowed him the power to take whatever decisions seemed right to him at the time.

Leninism meant that, unlike in most of the rest of Latin America, no one starved in Cuba and that all had access to a fine medical service and comprehensive education system, albeit a highly politicized one.

At the same time, dissent was savagely repressed, freedom to publish non-existent, homosexuality punished and challenges to Castro, his brother Raul and the Cuban Communist Party regarded as treasonable.

The ideology permitted Castro to encourage limited private enterprise one year and outlaw it the next. It allowed him to collectivize the sugar industry and most farms but allow private farmers to continue the tricky task of growing the world's finest tobacco leaves. It let him, who owed his education to the Jesuits, to rant against the Roman Catholic church and exclude believers from posts of influence at one moment and invite Christians to collaborate with him on his political projects the next.

The last pillar was the personal desire of a political leader with inexhaustible supplies of willpower and ambition to act on the world stage, leading the Third World in a campaign for a more just international economic order.

All three pillars are now rocking. The Third World, crushed by foreign debt, is concentrating its energies on avoiding starvation and has little strength left for campaigning.

Even in those few countries where there's still some strength left, the campaign plans are out of date and contradictory. Nicaragua, for instance, threatened by well-financed foreign aggression, has opted for a quiet life and a conservative government, while in Panama the tricky but often useful Manuel Noriega has been overthrown by the United States.

As the Third World lives through the crisis as best it can, Castro no longer cuts the figure he once did. From 1979 to 1983 his country occupied the presidency of the Non-Aligned Movement, but Castro's support for the Soviet invasion of Afghanistan lost him many friends in the Third World.

Today the concept of non-alignment is no longer what it was. Nevertheless, as Cuban troops return from duty in Africa, Castro and many other Cubans can remember with justified pride that but for his swift military action in Angola, some puppet of South African apartheid might today be ruling in Luanda.

Leninism, the second pillar, however, has been finally shown to be an ideological dead-end and an economic disaster, thrown off by the Eastern Europeans on whom it was imposed in the 1940s, abandoned by the very Russians who first developed it and beginning to be abandoned even by the Albanians.

The demise of Leninism as a militant creed whose followers were committed to making it triumph worldwide has also meant that Cuba has lost the value it once had as a political bridgehead in South America, a continent seen to be in the thrall of capitalism, colonialism and feudalism.

Moreover, the Soviets have neither the money nor the will to continue supporting Cuba in the style to which it had become accustomed.

There remains Cuban nationalism, the strongest pillar and the one in the best state of preservation. Castro remains the high priest of that very powerful doctrine. He has raised up Jose Marti, the 19th-century Cuban patriot, to a position in the Cuban pantheon equal to that of Marx or Engels. He continues to hurl anathemas at the Bush administration for its continuing economic blockade of the island and for the unceasing propaganda campaign that Washington directs daily at Cubans by radio and television.