In 2010 they didn't exist. Two years later they were a $50 million business empire and the biggest pop group sensation since the Beatles.
Indeed, according to many insiders, they've virtually already done so. Reports from the group's current US tour claim that things between Harry Styles and his bandmates are now so bad that they can't stay in the same hotels, travel in the same cars or write songs together.
It's claimed that problems stem from Harry's threat to quit the group and go solo. And we were told: "Harry thinks that the other four – Niall Horan, Zayn Malik Liam Payne and Louis Tomlinson – are immature and not so driven as him. Harry is the driving force behind the band and expects the others to take things more seriously."
One Direction only finished third in the 2010 X Factor contest but impresario Simon Cowell guessed he was on to a winner and signed them to a $3 million contract. It was money well spent. The group's first single What Makes You Beautiful? went straight to number one and their album, Up All Night was the fastest-selling album of the year and topped the charts in 16 countries. One Direction were the most successful boy band on earth.
It was a fairytale end to a story that started At the X Factor auditions when the five wannabes were first competing as solo singers without success. It was Simon Cowell's idea to make them into a group. He remembers "It took ten minutes to put them together", and Harry Styles came up with the name.
Now Simon, who was in at the beginning, is forecasting the end of One Direction if they can't sort out their differences – and quickly.
He says: "Bands don't stay together forever. I think they will make at least two more albums together after the current one. There are up and coming bands in the same genre and attracting much the same fan-base.
"Of course they are still a great band. They may have been manufactured but it worked. They were friends and they went in as equals and they were kind of fearless as well."
Now it looks as though the friendship is wearing thin. Journalists covering the current American tour report that Liam and Harry sometimes travel together but the others stay in separate hotels and order their own transport, including private jets. For instance, when the band was in Boston they all booked separate accommodation in hotels scattered around the city. "It seems they couldn't bear the sight of each other," said one observer."Later they left Boston in five different cars!"
Another insider claims: "Harry clashes with the others all the time about musical direction, particularly with Louis and Niall, who think he'll alienate fans with his plans to move away from pop. It's turning into a real power struggle but Harry's so stubborn. Nobody thinks they'll last after the next album."
Those close to the band say that Harry wants One Direction to move away from mainstream pop and ditch the clean-cut and so-called "Disney" image in favour of harder rock but Niall is particularly against this.
We were told: "Harry's plans were hatched after hanging out with stars including The Rolling Stones, Elton John and Rod Stewart, who have all urged him to take the next big step.
"Harry loves being in the band and the rest of the guys are his best mates but he is very ambitious and thinks they should be growing up more and developing a more mature style.
"Harry thinks their next album is the perfect time to create a more edgy rock and roll image and he's already writing songs that he wants to include on the album. The problem is the rest of the band fear that if Harry starts changing their style he's going to take over the whole thing.
"They all have different ideas on what direction they should go musically and Neill and Louis are especially worried about turning their backs on the band's distinctive sound and changing a winning formula.
"The fear is that if Harry doesn't get his way he could end up walking out and going solo – he's apparently had talks with several record companies about this. He doesn't want to be five years down the line and still doing boyish pop tunes."
Now Simon Cowell, who still has the band under contract, is planning a meeting to clear the air and as an insider says "maybe knock a few heads together. Simon still thinks there's potential in the group for some time yet.
"On the other hand, it's no secret that he's planning to organise a new boyband out of some of the latest X Factor hopefuls – just in case One Direction decide to call it a day."