When LeBron James was playing at St. Vincent-St. Mary High School, he almost always wore Adidas.

But when he entered the NBA at the age of just 19, he had switched allegiances to Nike.

A deal he now has for life. A deal which generates millions upon millions for Nike each year, with James twice as far ahead as the next most-popular player.

So where did Adidas go? Why did LeBron make the switch? Why couldn't Nike's biggest rival secure one of the most lucrative sponsorship deals in the history of sports?

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According to Sonny Vaccaro, a former sports marketing executive who was working with Adidas at the time, Adidas had a chance to land James with a 10-year, $100 million contract.

And it wasn't like Adidas couldn't afford it either.  They still had Kobe Bryant and Tracy McGrady selling shoes for them.

But Adidas lowered its offer to $US70 million at the last minute, allowing Nike to swoop in with a $US90 million deal to land the future superstar.

Vaccaro explained how it all went down in a podcast with US website The Ringer.

Listen to the full podcast here

"I wanted Adidas to give him $100 million, $10 million a year, guaranteed. $100 million is what it would have come to. And that number was off the wall. No one [had] ever gotten close to that [at the time]," Vaccaro says to The Ringer.

"We were going to bet our whole future on this kid, LeBron. There was no question that he wasn't going to be courted by other people because obviously he was going to be, but no one believed in him, not $100 million worth. That I do know.

"That number, I talked to [Adidas's owners], looked them in the eye ... and said, 'This is what I'm going to do.' And they said, 'Yes'.

"I never would have [offered $100 million] to LeBron James [if they hadn't said yes]. I mean, what advantage was that, to lie? They OK'ed it. So they knew, $100 million. I didn't spring it on them that day. They knew for nine months."

What happened next will forever burn in the memory of the Adidas company.

When it came time to wine and dine LeBron, they blew it, and they lowballed him with an offer that Vaccaro didn't see coming.

"So now we come down to the presentation," Vaccaro says.

"We bring him and his whole team on. Private airplane. We're going to get them [to] a Lakers playoff game. Imagine in Malibu, overlooking the Pacific Ocean, and we lay out the plan. And the number [was] supposed to be $100 million.

"I sat down that day [with] Gloria James, [LeBron's] lawyer, and ... LeBron, and all these people. I saw the contract. It wasn't $100 million. It was like $70 million, and they had incentives on it.

"It wasn't so much the number, $70 million or $100 million, because $70 million was a hell of a lot of money, right? But you have to understand what it was to me. The reason I was, and I still am respected, I believe, in that world, is [that] if I said something to you, you [would] believe me. If we had a deal, we had a deal. [Adidas] changed the number on me. I'll never forget that as long as I live."

But what would have really happened if LeBron had been offered the $100 million. Would he have taken it? Or was he already set on signing with the white tick?

"Well, I believe we would've, yeah," Vaccaro says.

"It's hard to say that now, because everything's gone so well. Do I think so? Yeah. Absolutely. Or I would've forced Nike to do $120 million, or $125 million.

"We went to a little corner of this mansion. Gloria, me, LeBron ... and I apologised. I'll never forget what they did. They put their arms around me and they said, "Sonny, we understand. We know what you did. We're going to be fine.

"I said then, I'll say until I die, the biggest mistake ever made in corporate America on this sort of a thing, was when Adidas backed out of signing LeBron James. [If] they sign LeBron James, the world changes.

"Nike was no. 1 before LeBron. Nike had great players. They always will. They were always no. 1 with the greatest personalities in sports. There's no question about that. I don't think that will ever change.

"My point to you is, [Adidas] could've changed the landscape.

"But my more important point: Even though Adidas has done well, they signed some really good players ... Sebastian Telfair, Dwight Howard, a lot of guys, but no one ever, since then, has ever come [close] to what James was. It was the dumbest, stupidest thing a corporate company ever did.

"I quit. I knew I was going to quit [Adidas] that day.

"Now [LeBron's] going to make a billion dollars at Nike before it's all over."