A HARD RAIN was falling in Lisbon last Wednesday night when the signal came for the drama to begin. It was a folk song, called "Grandola," broadcast over one of the local radio stations at half past midnight. By 6 a.m., Lisbon was completely surrounded by rebel troops; 12 hours later, Portuguese dictator Marcello Caetano surrendered, and the oldest fascist government in Europe was toppled.

The issue that brought the crucial impetus to end Portugal's 46 years of repressive rule lies several thousand miles away from the military maneuvering, in three African colonies where inconclusive wars have festered for 13 years. The implications of the sudden Portuguese move toward freedom for these colonies, and for all of Africa living under white minority rule, are immense.

Despite the euphoria now prevailing in Portugal, the prospects for real independence of the two largest African colonies--Angola and Mozambique--are probably quite small. The third colony, Portuguese Guinea or Guinea-Bissau, is in a different position.

The liberation fighters in Guinea-Bissau have been much more successful than the rebel forces in either Angola or Mozambique. And unlike Angola and Mozambique, Guinea-Bissau is not of strategic importance to the rest of white-controlled Africa, nor of much economic importance to Portugal.

The fate of all of the colonies will be determined to some extent--but by no means wholly--by the outcome of the political contest now going on in Portugal in the aftermath of the coup. The leader who has emerged most prominently is an unlikely liberal, Gen. Antonio de Spinola.

Spinola, a war hero from the stalemate in Guinea-Bissau, did not lead the army revolt but is credited with inspiring it by his publication last February of a book, "Portugal and the Future," which said the wars Lisbon was waging couldn't be won militarily and political solution was necessary. The 64-year-old cavalryman, who wears a monocle and carries a riding crop, rose to lead a junta after the officers who had begun the rebellion called on him to do so.

For a man who served as a fascist volunteer in the Spanish Civil War and an observer with the Nazi army, Spinola has taken some surprisingly progressive actions. He abolished the hated political police, known as the PIDE, a Portuguese acronym for International Police for Defense of the State. He has ended press censorship, established freedom of assembly and association, freed political prisoners, and promised free and open elections in a year.

But Spinola seems to have something less than independence in mind for the colonies. He told a group of newspaper editors Saturday that "self-determination should not be confused with independence" and has gone on record in favor of a democratic federation of Portugal and its African possessions, with each territory on an equal status with Portugal once the African people were "prepared for self-determination." That preparation, he said, would take some time.

THE NEWLY RETURNED leftists find Spinola's proposals for Africa inadequate. Although they advocate immediate cessation of fighting to be followed by full independence, the leftists are now apparently willing to move slowly in the elated atmosphere of freedom at home.

But while the debate in Lisbon continues, the dynamics of a new power struggle in Angola and Mozambique move on apace. Three African liberation groups fight a guerrilla war for control of Angola, and two operate in Mozambique. Although they now control little territory, if the Portuguese reduce their military presence, as seems probable, insurgent efforts to consolidate gains and take new ground must be expected.

In response to a strengthened insurgency, or perhaps in expectation of it, the white Portuguese settlers in Angola and Mozambique may decide to stage a preventive coup, similar to the one in Rhodesia, and install a white minority government not attached to Portugal. A desire for independence from Portugal among the white colonists--who number close to 10 per cent of the population--dates back to the 1950s, when many realized they could reap the benefits brought by a cheap labor force and rich resources without sharing them with Lisbon.

Now, if it appears that either independence or a greater degree of democracy is brought to the colonies--both of which would mean the displacement of whites from their exclusive hold on the higher positions in government and the economy--internal pressure in the white community for a Rhodesian solution will build.

At the same time an even stronger external pressure for white minority rule is likely. Angola and Mozambique are seen by the Republic of South Africa and Rhodesia as buffer states against a potential black African liberationist onslaught. These vast expanses of land--each well larger than Texas--thoroughly insulate South Africa from contact with any black nationalist government, and make infiltration and supply of potential antiapartheid guerrillas nearly impossible. This is one of the main reasons South Africa has entirely escaped any militant insurgency.

Rhodesia's interests are even more vital. Rhodesia is entirely landlocked--and virtually all of its exports and imports go through the Mozambican port of Beira. With Mozambique in black hands, and more than likely to exclude white-supremacist governments from the use of its resources, Rhodesia would have to ship all of its exports and imports overland through South Africa before getting to a port--the trip would be five times as long.

South Africa would be faced with a similar situation in regard to the Mozambican port of Lourenco Marques, which now carries much of the traffic to and from Johannesburg, South Africa's industrial capital. All of this cargo would now have to be routed through one of South Africa's own four ports.

The white supremacists have still more vested interests in sympathetic governments in Angola and Mozambique. The Cabora Bassa dam, the largest hydroelectric plant in Africa south of Aswan, is scheduled to be completed this year with its principal customers being industrial South Africa and Rhodesia. Another huge dam, on the Cunene River in Angola, is now being built with South African money. Black insurgents have been trying to stop both projects since their inception.

The Diamang-De Beers Company, a part South African concern, now controls the thriving diamond-mining industry in Angola. Presumably, an unsympathetic Angolan government would nationalize it. And some observers say that South Africa and Rhodesia are both relying on Angolan oil, coming from the Gulf Oil Corp.'s burgeoning Cabind a operation, to supply them in the event of an international embargo.

If a group of Portuguese settlers emerged who were intent on establishing their own white minority regime, they would no doubt find easy allies in Rhodesia and South Africa--allies that a weak and divided. Portugal, thousands of miles away could not resist. South African forces have secretly aided and trained Portuguese anti-insurgency units for the past six years. There is no reason to think that they would stop short of intervention if the situation became more threatening.