By JAY MAEDER Daily News Staff Writer

THE ASSASSINS from New York City had done no homework at all, and they had not the remotest chance of killing the President of the United States. The assassins from New York City appeared to think that in the nation's capital in the year 1950, you could just walk into Harry Truman's bedroom and shoot him as he slept.

Actually, there were two dozen armed guards and at least one submachine gun between the sidewalk and Harry Truman in the first place, and Oscar Collazo and Griselio Torresola were packing just two automatics and 70 bullets in the second place, and besides that, Oscar Collazo didn't know how to shoot his gun anyway. This was the situation when the assassins from New York City took the train out of Penn Station on Tuesday the 31st of October, their weapons in the pockets of the crisp new suits they had bought for the occasion.

They stayed the night in a cheap Washington hotel, and early the next afternoon they took a 40-cent cab ride over to Pennsylvania Ave. In the fall of 1950, the White House was undergoing renovations, and Truman and his family were temporarily ensconced in Blair House, the presidential guest quarters across the street. It was 2:20 p.m., and the sidewalk was crushed with tourists.

According to plan, what there was of it, the two determined attackers approached Blair House from opposite directions. Torresola paused in front of a sentry station. Collazo walked straight to the canopied steps, where a policeman was standing day duty, and he pulled out his gun and fired. There was only a click. Collazo didn't understand safeties.

In this one instant, there was now an end to any possibility that the assassins from New York City might actually get inside. Hearing the click, immediately grasping what it signified, the cop let out a yell and flung himself into the street to draw fire from the President napping upstairs. At this point, Collazo got his safety off and started shooting, but already hordes of guards were arriving from out of nowhere and down he dropped in a hail of bullets. At the same time, Torresola began blasting the sentry box. Two policemen fell, one of them forever. But as Torresola galloped toward the Blair House steps, dying Officer Leslie Coffelt got off one return shot that neatly drilled his killer straight through the brain.

Suddenly, in a few explosive seconds on Wednesday the 1st of November, it was all over.

The whole thing had been insane. "No one in his right mind would have tried to carry out an assassination like that," marveled a Secret Service chief.

Indeed, the attackers were fast identified as fanatics from Puerto Rico's Nationalist Party, revolutionaries dedicated to Puerto Rican independence and the overthrow of the island's U.S.-installed government. "Puppets!" raged the wounded Collazo from his hospital bed. In San Juan at this very moment, Gov. Luis MuÄnoz Marin's troops were rounding up nationalists who had days earlier mounted an insurrection that left dozens dead; soldiers were even now flushing party chief Pedro Albizu Campos from his stronghold. In Torresola's jacket, police found a letter from Albizu Campos appointing the dead man leader of the movement's U.S. operations.

Harry Truman was rather saddened. He liked to think he was the best friend Puerto Ricans ever had. Why would these men despise him so?

IN NEW YORK, the teeming Puerto Rican neighborhoods of East Harlem and the lower Bronx shook for days as federal agents and local police stormed the tenements in search of conspirators. It was widely imagined in Cold War 1950 that Puerto Rico's freedom fighters were Red dupes; the Communist objective, Daily Mirror columnist Victor Riesel explained, was "to divert us from Europe and the Orient and embroil us in Latin American uprisings to make us look like imperialist bandits, hated by our neighbors."

Collazo, 37, who had lived in the Bronx for more than 10 years and was otherwise a quiet $71-a-week workman in a New Rochelle handbag factory, had once been a commander in the nationalist military wing; Albizu Campos himself had once lived in Collazo's Brooke Ave. building.

It was true, of course, that U.S. sugar-company colonization had ravaged Puerto Rico's economy for decades and that the rabid nationalists were not entirely without a point. Yet Truman and his man MuÄnoz Marin were partnered in ambitious revitalization projects, and the President actively supported island self-rule, and it was not at all clear what ends Collazo and Torresola intended.

On Brooke Ave., it was generally observed that attacking the President really did very little to promote community interests, and neighbors began to shun Collazo's wife, Rosa, also a devoted nationalist. She made the papers for a few days, screeching party pieties - "Puerto Ricans can no longer be slaves of the U.S.!" - and then she fell silent.

FOUND GUILTY in March 1951 of Coffelt's murder and swiftly sentenced to death, Oscar Collazo eagerly looked forward to martyrdom, and he was annoyed when his execution was stayed, and then he was enraged when, in June 1952, on the eve of the implementation of the new Puerto Rican constitution that Truman had pushed through Congress, the President commuted his sentence to life behind bars. By this time, Pedro Albizu Campos was serving a long prison term in Puerto Rico. By this time as well, Rosa Collazo had softened. "A grand gesture," she called Truman's clemency.

Little more was heard of the imprisoned man until the mid-1970s, when a group called Fuerzas Armadas Liberacion National, or FALN, began blowing up banks and office buildings all over New York City. Among the FALN demands was the release of the hero Collazo, otherwise as consigned to the pit and abyss of popular memory as the presidential assassins Guiteau and Czolgosz and Zangara.

In September 1979, following a human-rights campaign spearheaded largely by Rep. Robert Garcia of the Bronx, Collazo was pardoned by good-hearted President Jimmy Carter, over the protests of Puerto Rican Gov. Carlos Romero Barcelo, who insisted that the aging convict remained a "menace to the public safety."

Humanitarian Garcia was soon disappointed by the newly freed Collazo's immediate public embrace of FALN and warm reendorsement of political violence. "I regret the words and language that are presently being used," Garcia admitted. Fidel Castro was not so chagrined. Two months after Collazo left prison, Castro called him to Havana and showered him with medals.

"As long as the imperialists are sure that they are struggling against a divided people," Collazo shouted wherever he went, "the empire will get the better of us."

SCENE: FEDERAL District Court, New York City, April 1980. An FALN freedom fighter named Marie Haydee Beltran Torres, charged in a 1977 bombing that killed a bystander, screams that she is a prisoner of war and does not acknowledge Judge Whitman Knapp's right to try her. As Knapp rules her position legally untenable and has her removed, she raises a fist in the direction of one courtroom supporter, elderly Oscar Collazo, and shouts: "Ni con carcel, ni con balas, esta lucha no la van a apagar!" This is a bumper-sticker revolutionary slogan that means: "Not with prison, not with bullets, the struggle won't be stopped." A New York police detective who happens to be of Puerto Rican heritage smiles. "I'm glad she's taking that position," he says. "Because it means she will definitely go to jail for a long time. It makes it easier to put them in jail when they say they are prisoners of war."

OSCAR COLLAZO died in Puerto Rico in February 1994 at age 80, no particular menace to the public safety. In a December 1998 plebiscite, Puerto Ricans rejected U.S. statehood and voted to maintain their commonwealth status.