The Beatles hit America like a seismic wave, changing the country and popular music forever. It would be hard to overstate the impact the band had in the US in the first months of 1964.
As US rock star Tom Petty has memorably described it: "The Beatles came on The Ed Sullivan Show and it was like the whole world changed overnight."
The band took America with dizzying speed. In 1963, while Beatlemania ripped through Britain, the Beatles hadn't been able to get even a sniff in the US. Then, on January 18, 1964, I Want to Hold Your Hand (their fifth British single) crept into the US chart at No. 45, starting a 15-week, five-million-selling run.
By February 1, it was No. 1. On February 7, the Beatles arrived at New York's John F. Kennedy airport to 3000 screaming fans. On February 9, more than 73 million people (45 per cent of the US population) tuned in to see their TV debut on CBS' popular variety program hosted by Ed Sullivan.
By April 4, they occupied the top five consecutive positions on the US singles chart and were in the midst of a 30-week run at the top of the album charts with three different albums.
And that was just the beginning. We all know what the Beatles mean to popular culture. They bestrode the 60s like a colossus and they resonate still - a band synonymous with the social and cultural upheaval of the baby-boom generation, who transformed pop music from inconsequential entertainment into one of the key art forms of our times.
Before they conquered America, Beatlemania was a British phenomenon. Afterwards, it gripped the entire planet. So why did they hit America so hard? And what might have happened if the Beatles had failed to impress the US?
The why isn't so hard to figure out. America was ready for a change and the Beatles were ready for America.
Ten years into the rock'n'roll era, the US pop scene was so twee that the biggest hit of 1963 was Dominique by the Singing Nun (a bespectacled nun, Jeanine Deckers, in her habit, playing an acoustic guitar). The raucous, rocking blues- infused music that had inspired the Beatles and other Liverpool groups had withered in its homeland - bowdlerised and repressed by an unspoken conspiracy of adult disapproval.
US commercial radio, in thrall to advertisers who demanded wholesome, family fare, was dominated by novelty records, cheery surf music and clean-cut male matinee idols with names like Bobby Darin, Bobby Vee and Bobby Bare. For the month of January 1964, boyish crooner Bobby Vinton held the No. 1 singles spot with There! I've Said It Again, a saccharine remake of an easy-listening 40s ballad older than most of its intended audience. This neutered pop aesthetic represented a last assertion of the paternal power held by the entertainment industry's old guard over the music of the young.
"Thank God for the Beatles," Jerry Lee Lewis later remarked. "They got rid of all the Bobbys, swept them aside like wheat before the chaff."
The truth is, pop in Britain before the Beatles wasn't in much better shape. But the Beatles were incubated in Liverpool and Hamburg, regional strongholds of rock'n'roll, with ears tuned to the hippest American imports, the upbeat new black dance music of Motown and the harmonic thrill of Phil Spector's girl groups. Beatlemania was built on a live fan base extending throughout England, and a slow-burning success with their fresh but simplistic 1962 debut single, Love Me Do. The rapid development of Lennon and McCartney's writing partnership ensured that each new release broke new ground and excitement spread like a wildfire.
Yet because of a turf war between Capitol Records and parent company, EMI, the Beatles' first four singles came out on tiny, regional independent labels in America (Vee-Jay and Swan) with no national marketing clout.
By the time Capitol relented, the Beatles were already firing on all cylinders, arriving in America with two albums and a host of the greatest pop singles ever heard. Britain got the Beatles by degrees. America got a full-frontal assault. The result was Beatlemania on another scale altogether and this, in turn, helped export the band to the rest of a world in thrall to postwar American cultural glamour.
And it wasn't just the Beatles. Their success opened the floodgates for the British Invasion, with a suddenly ravenous young American audience devouring all the new British groups whose potential had been unleashed. The artistry, style and adventure of groups such as the Rolling Stones, the Who, the Kinks and the Animals blew open the possibilities of a new, emerging rock culture while their American counterparts were still playing catch-up.
It is difficult to consider seriously what would have happened if the Beatles hadn't broken in America because it hardly seems possible. It was one of those rare occasions when the music itself seemed unstoppable, a force too great to be resisted.
It is surely questionable whether the Beatles would have made such rapid creative progress without the support of the biggest market in the world, and certainly their influence would have been diminished. Success imbues artists with confidence, it convinces financial backers to let them dictate their own course, and it inspires others to follow in their wake.
The latter might be the most important factor to consider. The potential of rock was certainly ripe to be exploited but who else could have led the way? The Beatles had set that revolution in motion in Britain and it seemed only a matter of time before someone crossed the Atlantic to unleash the latent promise of America.
Yet it seems unlikely the Stones, the Who or the Kinks could have achieved that kind of singular dominance. The breadth of their appeal set the Beatles so far apart from all their contemporaries - the contrasting yet complementary talents of two (and, belatedly, three) composers of charisma and genius (allied with band mates of skill, open-mindedness and personal compatibility) opening up so many musical avenues simultaneously - that they quickly transcended simplistic ideas of what a pop group could be.
Happily, the question is academic. On the first flight to America, Paul McCartney fretted: "They've got their own groups, what are we going to give them that they don't already have?" John Lennon was typically more bullish. Reflecting on their arrival in the US in 1964, he later told Rolling Stone magazine: "We knew we could wipe you out if we could just get a grip."