On June 28, 1971, Francis Ford Coppola was putting certain finishing touches to his costly, controversial adaptation of Mario Puzo's million-seller The Godfather. That day Coppola was shooting parts of the film's famous climactic massacre, in which Michael Corleone takes power of the New York mob by executing his rivals in a blizzard of machine gun-fire and Eisensteinian cross-cutting.

As Joe Spinell, playing one of Michael's button-men, pumped six slugs into a fictional New York mob boss trapped in a midtown hotel's revolving door, a for-real, blood-on-his-hands New York mob boss called Joe Colombo Sr, was being gunned down at an Italian-American rally in Columbus Circle, not four blocks away from Coppola's location.

The hit was the opening salvo in a vicious gang war declared by a newly released mafia upstart and criminal visionary named Joey Gallo. But it was the end of the strange connection between Colombo (who lingered in a coma until his death in 1978) and The Godfather, a movie that couldn't have been made without Colombo's say-so.

As detailed in C4's documentary The Godfather And The Mob (which borrows heavily from Harlan Lebo's The Godfather Legacy), Colombo had insinuated himself between the producer of The Godfather, Al Ruddy, and his own home turf of Little Italy, promising that the mob would take tribute from the movie, or not a frame of celluloid would be shot. Knowing that the movie would lose all its authenticity if shot on studio backlots, Ruddy had no option but to acquiesce, and once the media got hold of the story - a sit-down, handshake deal with the devil - they flayed him with it for months.

All this was, of course, great grist for the movie's publicity mill, and some commentators like Carlos Clarens, in his landmark 1980 study Crime Movies, recalled certain time-tested publicity-agent gambits: "the filmed-under-threat routine had worked wonders back in the days of Doorway To Hell (1930: Jimmy Cagney's second movie)." If nothing else, Lebo's book and The Godfather And The Mob prove beyond a doubt that none of this strange tale was concocted by press agents.

The details are toothsome and delectable. The Godfather was written by Puzo, an Italian-American who grew up in Hell's Kitchen but who had never met a bona-fide mafiosi. Puzo learned his mob folklore mainly from croupiers in the golden age, 1960s Las Vegas of Moe Dalitz and the Rat Pack. That didn't prevent him from achieving such an impressive degree of authenticity that by the time the movie was a runaway hit, many real-life mafiosi had begun comporting themselves according to the rituals solemnised by Puzo and Coppola - the cheek-to-cheek kisses, the quasi-papal pledging of fealty to the Godfather's ring.

The total-immersion experience of the movie - achieved by the goldfish-bowl effect of keeping the audience emotionally intimate only with mobsters, by the subterranean browns and golds of its colour scheme, and by its period, ethnic and socioanthropological authenticity - traps us in 1945, and even now it is hard to imagine that a block away from the border of the set, it was 1971 and the real New York mob was undergoing the same upheavals as everyone else in those Martian times. Although The Godfather And The Mob hints at much of this, it has no real grasp of the richness and complexity of this period in mafia history.

Colombo was the head of what had earlier been the Profaci crime family, which he had inherited in the mid-1960s only because Joey Gallo was in prison for 10 years.

In Goodfellas' famous circularshot of teenage Henry Hill's "introduction to the world" in 1955, Hill's narration says, "It was a glorious time, before Appalachin and before Crazy Joe started a war with his boss ..." Appalachin referred to a famous FBI raid of the upstate New York estate of a leading crime boss in 1957. A mob summit was taking place and agents chased dozens of top mafiosi through the snow as they dumped guns, jewels and thousands of dollars in cash (the incident is alluded to in the final episode of season five of The Sopranos, as Tony escapes the Feds, but New York boss Johnny "Sack" Sacrimone does not).

Joey Gallo, meanwhile, saw drugs as the coming bonanza for organised crime and in the teeth of stiff opposition from the abstemious old "Moustache Petes" of the Corleone/Lucky Luciano generation, he had no compunction about forging distribution partnerships with black criminals in Harlem and Bedford-Stuyvesant and shipping major product.

The war that ensued in the late 1950s (obliquely alluded to in Godfather II - "Not here, Carmine!"), tore the mob apart, grabbed headlines, and encouraged new Attorney General Bobby Kennedy to prosecute the mob unmercifully after 1960 - focusing on such figures as Teamsters Union boss Jimmy Hoffa, and the mafia bosses of Chicago, Tampa and New Orleans (who may later have helped assassinate his brother John). So it was an exhausted, much harried New York criminal fraternity that greeted Coppola and Ruddy in 1971.

It was also a community that had little taste for publicity. At the movies, the words "mafia" and "cosa nostra" were rarely ever heard before The Brotherhood in 1968 (which sank faster than Johnny Rosselli in his concrete-filled oil-drum). Even J Edgar Hoover downplayed the importance of the mafia throughout the 1930s, 1940s and 1950s - while exaggerating the moribund red menace - probably because the mob's financial genius Meyer Lansky (Hyman Roth in Godfather II) had, presciently, blackmailed Hoover over his homosexuality as early as 1935.

Still, in an era highly conscious of matters racial and ethnic, Italians like Joe Colombo found a way to express their sense of ethnic grievance, too. Although the Italian community was well served by social groups like the Knights Of Columbus and the Order Of The Sons Of Italy, Colombo became involved in a new outfit, heavily mob-influenced and called, in the spirit of the times, the Italian-American Civil Rights League. And The Godfather's arrival in Manhattan gave the group a chance to raise its profile.

The league demanded consultation rights and got them from Ruddy in exchange for access to locations. Frank Sinatra - probably not pleased at Puzo's oblique references to the manner in which he secured his comeback role in From Here To Eternity - headlined a league fundraiser at Madison Square Garden, and local politicians attended the league's first rally in 1970, decrying anti-Italian prejudice (one hears the echo of Joe Pesci's plaintive wail in Goodfellas: "She's prejudiced against Italians. Imagine that - a Jew broad!").

They had a point - up to a point: Gangsters in the movies before 1970 were redolent of grotesque and venerable stereotypes about unwashed Italian immigrants pouring off Ellis Island. On the other hand - tell it to Sidney Poitier.

Or consider a contemporary figure like Anthony Imperiale, "the White Knight of Newark", namechecked by Tony Soprano in series four. Imperiale rose in the aftermath of the 1967 Newark riots as a streetcorner agitator exploiting Italian-American fears about black encroachment on hitherto white neighbourhoods - which he patrolled after dark with carloads of excitable, albeit unarmed young men.

Imperiale disavowed any racist intent, indeed he merrily hijacked the language of the real civil rights movement, despite talking of "Martin Luther Coon" and invoking a feral, spectral "them" whenever he mentioned blacks. You can breathe this toxic atmosphere of neighbourhood insularity and racism throughout Robert De Niro's A Bronx Tale, also set in those years.

A hunger for headlines and flashbulbs seemed to be part of Joe Colombo's motivation in entangling himself with the league and the Godfather shoot. It was to be his undoing. His secretive, camera-phobic criminal cohorts got fed up with him. Working in partnership with capo di tutti i capi Carlo Gambino, Joey Gallo, free again and no less crazy, had a black criminal associate, one Jerome Johnson, gun Colombo down at the Italian-American League's second annual rally at Columbus Circle.

A black triggerman in a mob hit was then unheard of, and totally alien to the mafia's modus operandi, but no one was fooled. Johnson was gunned down in seconds by an assailant who immediately vanished, but everyone suspected Gallo because of his Harlem connections.

By the time Gallo himself was killed a year later - gunned down in a Mulberry Street clam house while celebrating his 43rd birthday - he had acquired his own taste for publicity: he was feted by writers (he'd read Camus and Sartre in the can), and was pimping his own memoir, A-Block. After Joe Colombo's fatal experience with The Godfather, you'd think Gallo might have learned his lesson. As it turned out, he died the same way as Virgil "The Turk" Sollozo at the hands of newly-minted murderer Michael Corleone, in an explosion of blood and clam sauce - just like in the movies.

· The Godfather And The Mob, Tue, 11.05pm, C4