Just after midnight on a night in September 1974, Chilean Gen. Carlos Prats and his wife, Sofia, died on a Buenos Aires street in a burst of machine-gun fire and a powerful explosion.

At the time, their murder - which came almost exactly a year after the bloody overthrown of Chilean President Salvador Allende - was almost lost in Argentina's widespread violence.

Nonetheless, it may have been the most significant killing of the Chilean coup except fort the death of Allende himself, for Prats had been a principal obstacle to the coup and, afterward, a permanent potential threat to the junta.

Now the unexpected publication here in Mexico of a private diary kept by the murdered army commander is arousing more interest than just about anything else written about the Chile's socialist experiment and its bloody consequences.

Commander-in-chief of the Chilean army and, at one time or another during the three Allende years, interior minister, defense minister and vice president, Prats was the one military man who could have challenged the junta members on their own ground and, in addition, have drawn civilian support. Many people, in fact, were hoping that Prats would be the next president of Chile.

The 137-page paperback - called "A Life Within the Law" (Una Vida Por la Legalidad) - has an anonymous prologue and offers no photographs of the manuscript or any other attempt to deal witht he inevitable question of authenticity. Many of Prats' friends knew that he was working on a book on recent events in Chile and the armed forces' role, but apparently no one knew of this diary.

If the text is to be taken as the author's word - adn there seems no real reason not to - the diary was kept as an aide-memoire to his other writing. The book manuscript itslef disappeared immediately after his death; intimate friends of the Prats' say it presumably was kept by Chilean embassy officials who were the first to enter the murdered couple's apartment.

At the publishing house here a spokesman said he understood that the diary had been smuggled out of Chile, but he said he could say no more at the "explicit request of the person who placed it in our hands."

"We have no doubt about its authenticity, though," he added.

With much of the Allende years, events now duly unraveled, the diary has few stunning revelations. What this critical, often-moving account brings to the event is the unique perspective of a thoughtful, accomplished military man who was able to operate in and between the military and political words.

The diary begins in February 1973, when Prats is "overwhelmed" by the problems of the Interior Ministry and the Cabinet and his duties as army commander. The entries reply primarily on first-hand experiences and observations. Sometimes he stops to analyze the turmoil in his society, other times he offers interesting political insights by recording his conversations with the president.

For example, Allende recounts to him a meeting he had with then-U.S. Ambassador Edward Korry, who advised him to reach an understanding with the United States "which would have meant renouncing the nationalization of copper. The president told me he had to cut the ambassador off abruptly when he tried to accompany his suggestions with threatening insinuations."

Above all, Prats blames Chile's own politicians for bringing about the "holocaust": the opposition for sabotaging the economy and conspiring with the military, the government for paralyzing itself with its own internal disagreements.

But Prats is also keenly award of U.S. intervention. On Aug. 27, four days after rebelling generals have forced him out of office and two weeks before the coup, he writes:

"I believe we have underestimated the seriousness and the extent of the conspiracy manipulated from the United States . . . althought we knew of Kissinger's private meeting in Chicago two weeks after the Unidad Popular [Allende's leftist coalition] won the elections." At that meeting, Kissinger said plainly that the United States would not accept the possibility of a Communist government in Chile, Prats writes.

Most interesting, perhaps, are his observations about Chile's armed forces, in which he spents his life and whom he knows "as my own children."

"I belive that neither President Allende nor the government parties know how profound the North American influence is in our armed forces, and especially in the mantality of the Chilean military man," he says, calling it an "influence without counterweight that I wanted to limit or at least balance."

He was convinced that Chile's military, inevitably becoming politicized, ought to understand the country's social realities and participate in the tasks of developing it. He told the president that officers should be sent to Europe, Africa and Asia - "not to copy them, but to understand that the world does not begin and end with the schools of the Pentagon."

But in the School of the Americas and other Pentagon organizations, he writes, Chilean officers learned instead to respond "to the stereotypes and reflexes of those courses." "While thinking that they were liberating the nation from 'the enemy within," Prats wrote, "they have committed a crime that can be explained only in terms of their ingenuousness, ignorance adn political shortsightedness."

proof of the "simplicity" of Chilean military thinking, he told this reporter in an interview after the coup, was - that "Our military intelligence exclusively investigated activities and terrorism of the left, while the right was just as dangerous and strockpiling more arms."

Appriasing the diary against the background of long conversations that I had with Prats after the coup, in his Buenos Aires exile, it is consistent with many of the ideas and opinions he expressed. But some topics do not appear in the text - which seems surprising, judging bu the intensity with which he discussed them at the time.

One such topic is the U.S. military role in the months leading up to the coup. he admitted that he did not know all its details, but he harped on the fact they "coordination and preparation for the coup" had taken place in the port of Valparaiso, where high Chilean navy officials "plotted directly with representatives at the U.S. naval station."

Another point was his virtual obsession with the "destructive tactics" of Eduardo Frei, Cile's Christian Democratic president from 1964 to 1970. More than Allende, whom Prats saw as well-intentioned but unable to control his own parties, he considered Frei responsible for the coup. Frei was using the military to get back into power himself, prats said, and when he had gone to plead with the former president for peace and reconciliation with Allende, Frei had been "arrogant and insulting."

Prats said he had been particularly shocked to discover Frei's hand behind "Operation Charley," the campaign designed to discredit him and force him out of his command. The campaign pushed by the Christian Democrats, he said, had access to anti-Allende funds "provided by the Italian and West German Christian Democratic parties."

After the coup, when Prats got a job as an accountant in a Buenos Aires tire factory, he seems to approach despair as he learns of the "executions, tortures and acts of corruption" perpetrated by an institution to which he had dedicated most of his life.

Gen. Augusto, pinochet, a man he knew all his life, who led the coup and became president of Chile, leaves him dumfounded. Prats had believed him to be a loyal constitutionalist, and repeated this to President Allende as Pinochet succeeded him.

As time passs, Prats writes, Pinochet proves to be "a villain of limited capacity and unlimited ambition" who spent a lifetime "being servile and crawling, waiting for the moment to commit a cowardly crime."

" I am convinced he joined the band-wagon of the coupmakers only at the last minute, but I have no doubt that he will hold on to power at any cost. He will be known as the great traitor of our history," Prates writes.

,ost moving are the passags where he describes the humiliations and deaths of his comrades-in-arms and deplores the destruction caused by men he knew well, who have "demolished, vilified, soliled everthing" and even "turned the word 'Chile' into a curse before the world."

As Prats realizes he is being followed by "agents of the Chilean juntat" and death threats increase, he presses the Chilean consul in Argentina to give him the passport he has vainly been requesting for seven months.

He plans to travel to Europe: "Sofia is suffering from this war of nerves." He told friends he had finished almost 400 pages of the book, which is still missing.

Unlike his colleague in the Cabinet, Orlando Letelier, who was murdered last year in Washington. Prats never spoke out publicly or acted against his former companions.

The last entry in the diary, six weeks before his death, may indicate why someone decided he had to die: Chilean military men were contacting him.

He mentions a despairing letter written by two offiers and a sergeant's offer to leave retirement and go into exile in Argentina "if you need me."

"This is not the first message that comes from within the armed forces." he writes.

His reply to the sergeant's brother, who brought the messaeg, was: "Thank you very much. Stay where you are. Your duty is to help save the army from the situation it has been dragged down into."

The diary, sometimes written in the melodramatic rhetoric of official Spanish and sometimes with direct simplicity, shows Prats as many people - friends and critics - knew him: a decent man, caught in a tragedy he had struggled to prevent.