It was 50 years ago today that The Beatles flew across

They came, they sang, they conquered. And 50 years after four young men from Liverpool with rebellious "mopcut" haircuts and a cheeky swagger set foot on US soil for the first time, America will for the next two weeks embark on a golden anniversary spree of Beatlemania.

Even on the transatlantic Pan Am Clipper flight to New York, John Lennon had wondered whether the Beatles were really going to make it big in America, a market that no British performers had previously captured. The tumultuous scenes that greeted them at Kennedy airport on February 7, 1964 quickly put those doubts to rest.

Two days later, 73 million viewers - a near-unimaginable 40 per cent of the country - tuned in to see the US debut on CBS's Ed Sullivan Show of a group that had been almost unknown there a few weeks earlier.

I Want To Hold Your Hand, their first major label single in America, had soared unexpectedly to the US number one spot after its Boxing Day release.

By early April, the Beatles would occupy the top five places in the Billboard charts - a feat never achieved before or since.

The barrage of US television specials, fan festivals and tribute concerts begins this evening with a rare reunion of the two surviving Beatles - Sir Paul McCartney and Ringo Starr - at the Grammy awards, where they will be honoured belatedly with a lifetime achievement award.

Then on tomorrow night they will play in a star-studded line-up alongside the likes of Annie Lennox and Alicia Keys in a Beatles tribute concert show being taped for broadcast on the February 9 anniversary of that Ed Sullivan Show.

The celebrations are about much than more simple nostalgia. In America, the Beatles are not just revered as the most influential and ground-breaking performers in popular music but also as four young men who exerted unprecedented sway over social mores, political passions, cultural trends and spiritual ambitions.

They were also the exuberant foursome who seared themselves into American souls as they helped a nation emerge from the gloom of its darkest post-war era - the assassination of President John Kennedy less than three months earlier.

"The Beatles represented something the country desperately needed - a diversion, just 77 days after the assassination of Kennedy," recalls Marvin Scott, then a young television news reporter who covered their arrival at the airport newly named in honour of the murdered president.

"Beatlemania was also a welcome respite from news of the Vietnam War, and the civil rights battle in the South." Mr Scott, now the senior correspondent with New York's WPIX-TV News, was among the journalists wondering what all the fuss was about as they awaited Pan Am flight 101.

But the piercing screams of the fans on the observation deck as the plane came into view quickly made clear that this was a special day.

"There was a crescendo of 'We want the Beatles, We want the Beatles', and more shrieks when the doors of the plane opened and the four emerged waving to the crowd. They were a strange looking lot, I thought. What with those funny-looking mod suits and pudding bowl haircuts, they could have been aliens from another planet. Beatlemania had come to America. There was pandemonium.

Fifty years later, I can still hear their screams ringing in my ears." Martin Lewis, a British writer and Beatles scholar, has chronicled how the band went from virtual unknowns to superstars in just six weeks thanks to timing, chemistry, luck, and, of course, talent.

"American rock and roll had inspired the Beatles in the 1950s, but by the early 1960s, the US music scene and fashion was bland and predictable and old-fashioned," he said "The Beatles blazed a trail.

You just have to look at the footage from the Ed Sullivan Show. There are the boys in all their vibrant and cheerful glory, so exuberant and effervescent and cocksure, and then the camera pans to the audience and they look like they are dressed for a 1950s dance hall.

"Other bands would have stood still, doing the same thing, just learning to do it better. But the Beatles broke the rules. They were doing new stuff and better stuff. They defied the rules of celebrity physics - that after you come up, you go down. They never faded away."

Walter Egan, a veteran American rock musician, was a 15-year-old who had just formed his first band when he attended the Beatles' first concert in New York at Carnegie Hall on February 12. "The place was full of girls and their screams were so intense that I could really only hear the bass guitar," he recalls.

"It was perhaps the most frustrating and the most amazing concert I've ever been to. The energy and the atmosphere were fantastic and it felt like we there at the start of a musical revolution."

Egan, who will perform in a tribute concert on February 8 as part of the New York Fab 50 programme, had bought his copy of I Want To Hold Your Hand just five days after its release and watched the band play on the Ed Sullivan Show.

"They were so exhilarating, it felt like a changing of the guard. To me they were the coolest people in the world. They were the frame for the pictures of the 1960s." The bald numbers tell the story of the bestselling act in history, with more than 600 million albums sold worldwide and 20 number one singles.

Half a century later, their catchy melodies are as instantly recognisable now as they were revolutionary then. It is a success that not even the cocksure Liverpudlians could have imagined as they prepared for that trip. "Oh, yeah, we thought a couple years, that would be it," Sir Paul said in 2009.

"We never thought it would last at all." Their talent and commitment were supreme, but they were also favoured by timing, coincidence and circumstance - in their arrival in the US as much as their acrimonious split just six years later.

"As a group, the Beatles died young and left a perfect corpse of incredible music," said Mr Lewis. "We never had the embarrassment of the Beatles doing disco. It wasn't planned but they quit at their peak and it was their final great move."