Chael Sonnen explains why he likes the idea of co-promotional matchups, but these fights won't be exciting until the UFC gets involved. (1:34)

Scott Coker was a neophyte mixed martial arts promoter in 2006. His company Strikeforce was in its infancy, coming off its inaugural event.

Coker had a card scheduled in the promotion's home base of San Jose, California, for that June, but it did not feature a fighter who could sell a lot of tickets. After decades in kickboxing, Coker was trying to grow Strikeforce into a player in the MMA game. With that in mind, he contacted Nobuyuki Sakakibara, then the promoter for vaunted Pride FC in Japan. Coker and Sakakibara had a relationship from Coker's time working with K-1 kickboxing.

Coker made a huge ask: Could Pride send Strikeforce a main event-level fight for the San Jose show in return for future talent sharing? Sakakibara agreed and sent a bout better than Coker could have ever dreamed: Vitor Belfort vs. Alistair Overeem. The two men were among the most well-known fighters on the planet at the time.

Thirteen years later, Coker and Sakakibara are still swapping talent, still cross-promoting in a significant way. On Friday night, Sakakibara will send Kyoji Horiguchi, his Rizin bantamweight champion, to fight for the Bellator title against champ Darrion Caldwell at Bellator 222 at New York's Madison Square Garden.

Caldwell traveled to Japan on Dec. 31 to face Horiguchi on Rizin's big year-end show in a bout that was the inaugural Rizin title fight at that weight class. Horiguchi won by third-round submission. It was always the agreement between both sides to do the rematch in Bellator. And if Horiguchi wins the Bellator belt this weekend, he will be obligated to defend it at least once a year.

Coker sold Strikeforce to UFC parent company Zuffa in 2011. After his non-compete agreement was over, Coker was hired by Bellator. Sakakibara has had a similar path; he sold Pride to Zuffa in 2007, then started Rizin after his non-compete was completed in 2015.

Through all of that, the relationship between Coker and Sakakibara has remained strong. Coker will never forget what Sakakibara did for him in 2006 when Coker had almost nothing to offer in return at that juncture.

"That's why when he came back from his hiatus from Pride into Rizin, I said I'm gonna support him," Coker said. "Because when I was first starting, he was there for me and really helped me out and sent over two of the biggest names in the sport at the time."

Coker was able to reciprocate shortly thereafter. In August 2006, Gilbert Melendez went to Pride to face Nobuhiro Obiya, then he fought Tatsuya Kawajiri at Pride Shockwave 2006 on New Year's Eve of that year. Melendez won both.

Sakakibara told ESPN he would have sent fighters over to Strikeforce even if Coker could not send him any compelling athletes back right away.

"He had the love of the sport, he wanted to take the sport to the next level and build his brand," Sakakibara said through translator and Rizin executive Shingo Kashiwagi. "The UFC was already a big player in the U.S., and any business needs a healthy competition, so ... it was necessary to help [Coker]. It was possible because of the mutual trust and respect that we had between us."

Cross-promotion and talent sharing is a big part of Rizin's philosophy, which is why Sakakibara said he named it Rizin Fighting Federation. "Federation," by definition, is a group of entities. Sakakibara's ideology is that MMA will be able to advance only if promotions work together. His end goal is a World Cup of MMA every four years, where all of the big companies' athletes compete to truly determine the best fighters in the world. He would like to get the athletes' countries involved in directly supporting them.

Sakakibara said the UFC is currently "isolating itself" from other promotions around the world and, by doing so, not allowing MMA to get to where he believes it could. He acknowledges, though, that it would be a risk for the UFC to collaborate, considering the successful structure the promotion has built.

"Obviously, the UFC has done a fantastic job of spreading this sport, making this sport recognized and established," Sakakibara said. "There's nothing to take away from what the UFC has done to build the sport. ... The UFC has MMA already established, and it won't go anywhere. But it won't get to the kind of level where soccer is. It won't get to that level unless we get all countries involved, like the Olympics. We won't be able to make it into a world-accepted sport. That is how I feel."

Kyoji Horiguchi, right, defeated Darrion Caldwell in a matchup between Rizin and Bellator fighters on Dec. 31. Etsuo Hara/Getty Images

There was a time when the UFC was open to cross-promotion. The UFC sent one of its big stars, Chuck Liddell, to Pride in the early 2000s, most notably for the Japanese company's 2003 middleweight grand prix tournament. Liddell beat Overeem by first-round knockout in the quarterfinals and was knocked out by Quinton "Rampage" Jackson in the semifinals.

Since then, the UFC has done talent sharing sparsely. The UFC and Mayweather Promotions did team up on the Floyd Mayweather vs. Conor McGregor boxing match in 2017, which was one of the biggest money making pay-per-view events ever (4.3 million buys in North America) and led to record revenue for the UFC that year.

It was a bit different, but the UFC worked with ONE Championship last year on a trade, sending its former flyweight champion, Demetrious Johnson, to ONE for former ONE and Bellator champion Ben Askren. UFC president Dana White told TSN's Aaron Bronsteter over the weekend that there aren't any other fighters out there currently that have gotten his attention.

"We just did this thing with Askren," White said. "But no, I don't see that. Askren was a guy that I was interested in. I don't see anybody else out there right now that I'm interested in. But if I did, anything is possible."

There is considerable risk in doing cross-promotion. MMA companies pour resources into building up and marketing fighters. If one of them loses to a competitor's athlete, it's a knock against the organization. Plus, there are considerable other obstacles to true cross-promotion, such as television deals and compensation. Coker said Bellator will pay Horiguchi according to his Rizin contract on a handshake deal; it will not count against the amount of fights he has left with Rizin.

Cross-promotion with Rizin has been a mixed bag in the win-loss column for Bellator. Bellator star Muhammed Lawal won the Rizin heavyweight grand prix tournament in 2015, then was knocked out by Mirko Cro Cop in 2016 and Jiri Prochazka in April, leading to his retirement.

"You're gonna win some, and you're not gonna win some," Coker said. "As long as my fighters want to do it, our schedules work out -- we both have businesses to run and we're very busy -- but if we can make it happen from time to time and have a lot of fun with it, why not?"

Lawal is a huge proponent of these kinds of deals. He thinks the exposure fighters get from them is important.

"There's more talk about the fighters and not the promotions when it happens," Lawal said.

Horiguchi said Rizin's cross-promotion with Bellator is helping to grow the Japanese MMA scene.

Caldwell said he is hoping the UFC will agree to a cross-promotion, to allow him to fight some of his fellow top bantamweights, such as new UFC champion Henry Cejudo.

"People want to see Darrion Caldwells versus the Cody Garbrandts and TJ Dillashaws," Caldwell said. "They want to see the Douglas Limas versus the [Kamaru] Usmans. They want to see it. I think it's just gonna bring more fun, more fans and more excitement to the sport."

Horiguchi has his hands full with Caldwell on Saturday, but he said he would like to compete in the future with the best in the UFC. Most of all, the Japanese striker desires a future date with former UFC flyweight champion and current ONE Championship star Demetrious Johnson, who beat Horiguchi at UFC 186 in 2015.

"I need to return the favor of delivering a loss," Horiguchi said through a translator. "So yes, sometime down the road, I really do hope that I get to fight him again."

Bellator president Scott Coker has long been a fan of cross-promotion in mixed martial arts. David Fitzgerald/Sportsfile/Getty Images

That's the kind of thing Sakakibara is talking about -- the best fighting the best, regardless of organization. Rizin is sending one of its women's stars, Rena Kubota, to Bellator this weekend to fight Lindsey VanZandt. Invicta FC atomweight champion Jinh Yu Frey fought her counterpart Ayaka Hamasaki earlier this month at Rizin 16. And Rizin's Kanako Murata made her Invicta debut last week. Meanwhile, Coker said he expects Bellator to be in Japan for its show sometime in the next 12 months. And Bellator and Rizin might swap talent for their respective upcoming tournaments.

Sakakibara seems to be making inroads toward his ultimate goal of a World Cup of MMA, though it will be hard to get the UFC on board. Cross-promotions are not easy to do, Coker said. There are a lot of moving parts, and a strong relationship with other people is key.

"Basically, you have to have a certain amount of, let's say, trust in another promoter and be willing to work with them, and they'll have to work with you," Coker said. "And you'll have to be able to get along to make all this happen. That's quite difficult sometimes, because everybody has their own business to run and their own agenda."

For 13 years, that trust has existed between Coker and Sakakibara. In 2019, cross-promotion is alive and well in MMA, if Bellator 222 is any indication. Perhaps there will be more to come.