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When Errol Spence Jr. and Terence Crawford step into the ring on Saturday night, they'll do so while making history. Throughout all of boxing history, there have only been 12 previous unification bouts at welterweight, but most of those clashes have been historic and memorable affairs. Spence and Crawford are fighting to crown the first undisputed welterweight champion in the four-belt era.

Spence enters as the WBC, WBA and IBF champion while Crawford brings his WBO title to the fight. Both men are undefeated in their impressive careers and occupy high places on every legitimate pound-for-pound ranking in the sport. This is a fight that has been talked about for years and is now finally happening as arguably the most important fight of 2023.

While the list of welterweight title unification bouts is short, the fight with Crawford will be the third time Spence's name has been placed on it. Spence unified his WBC title with the IBF when he defeated Shawn Porter in 2019. He then added the WBA title by beating Yordenis Ugas in April 2022.

This is Crawford's first unification bout at welterweight, but he fought in two during his time at junior welterweight, where he eventually became undisputed four-belt champion.

With that in mind, let's take a look at five of the biggest welterweight unification bouts in boxing history.

"Sugar" Ray Leonard vs. Thomas Hearns

Having avenged his loss to Roberto Duran and regained the WBC crown a few months shy of a year earlier, Leonard found his way back into the ring with another member of the "Fabulous Four" on Sept. 16, 1981 for a fight dubbed "The Showdown." Hearns was long and lanky and hit with tremendous ferocity. He'd won the WBA championship with a knockout of Jose Cuevas in August 1980 and had defended it successfully three times with each win coming by knockout.

The fight started slowly but ramped up in intensity as the rounds wore on. Leonard answered questions about his chin, taking the heavy shots of Hearns, but swelling around his eye showed the power of Hearns was taking a toll. While Leonard wasn't expected to be able to punch with Hearns, he did so more than capably, though Hearns turned up the heat and began picking up rounds down the stretch, eventually adding up enough rounds where Leonard could at best battle to a draw unless he could score several knockdowns or earn a stoppage.

Between rounds 12 and 13, legendary trainer Angelo Dundee told Leonard, "You're blowing it now, son. You're blowing it." In response, Leonard upped his volume and blasted Hearns in the 13th, badly hurting him in the mid part of the round before scoring a knockdown in the final seconds. Round 14 was more of the same with Leonard hurting Hearns and pouring on unanswered punches, finally leading the referee to halt the contest and award Leonard the win in a fight that would not have gone his way on the official scorecards.

Oscar De La Hoya vs. Felix Trinidad

De La Hoya vs. Trinidad was one of the biggest fights in boxing history. Both De La Hoya and Trinidad came into the bout as undefeated welterweight champs, De La Hoya having won the WBC belt in a mauling of Julio Cesar Chavez and Trinidad taking just two rounds to wreck Maurice Blocker and capture the IBF championship. De La Hoya had become an international superstar while Trinidad earned a reputation as a divisional great with 14 defenses of his belt coming into the bout. 

The 1999 clash was labeled "The Fight of the Millennium" and drew 1.4 million pay-per-view buys and $12.9 million at the gate, both numbers records outside the heavyweight division. Trinidad would take the win in what is still a highly debated decision both for how the fight was scored and De La Hoya's approach.

After building a small lead over the first nine rounds, De La Hoya began to fight off his back foot to protect a lead not as large as he and his corner had calculated. With Trinidad turning up the heat, and having to only really avoid De La Hoya's jab, the Puerto Rican slugger clearly took the final three rounds. The late swing in momentum allowed Trinidad to take the majority decision by scores of 114-114, 115-113 and 115-114 and left fans and media to debate the defensive strategy of De La Hoya and the scoring of the opening four rounds for decades.

Vernon Forrest vs. Ricardo Mayorga

"Sugar" Shane Mosley was supposed to be another poster boy for boxing. A near invincible force at welterweight with a TV-ready smile just like De La Hoya. But Vernon Forrest came along and beat Mosley twice, capturing the WBC title and seemingly launching himself into the discussion of the elite of the elite in the boxing world. Fresh off his two wins, over Mosley, Forrest signed a big six-fight deal with HBO and was placed into what many figured was a showcase bout with little-known Nicaraguan wildman Ricardo Mayorga. But, as George Foreman would say on commentary during the fight, "Boy, you sign those big contracts and the last thing on your mind is that you gotta fight for the money."

Mayorga came from a tough life, his body covered in scars from knife fights as a gang member, his head scarred from a lead pipe. He came up through the boxing ranks despite a pack-a-day smoking habit, largely because he punched with a power few chins could hold up against, leading to his claim of the WBA belt from Andrew "Six Heads" Lewis. His determination and ferocity met Forrest, who was accompanied to the ring by a full crew, including a pair of rappers when they clashed on Jan. 25, 2003. The supremely confident Forrest had four inches in reach on Mayorga and was a 6-1 favorite at bell time.

Forrest easily outboxed Mayorga in the first round, only to lose the round after a late flurry from Mayorga put him on the canvas. Forrest obliged Mayorga in the second round, engaging in an increasingly wild brawl that led to Round 3 and a massive right to the temple that dropped Forrest again. Forrest got to his feet but was clearly in no shape to continue, giving the underdog the shocking knockout victory. As Mayorga celebrated, HBO's Larry Merchant summed up Forrest's performance, "Let's see Vernon Forrest come into the ring rapping the next time," he said. "He was celebrating his championship before he had a serious chance to defend it."

Floyd Mayweather vs. Manny Pacquiao

A fight that seemed inevitable, both because of the caliber of fighters involved and the mountains of money to be made, took more than half a decade to put together. But fans would eventually be treated to "The Fight of the Century" on May 2, 2015 when Manny Pacquiao and Floyd Mayweather stepped into the ring against one another.

The undefeated Mayweather was a five-division world champion and had won the WBC title from Victor Ortiz six fights prior to stepping into the ring against Pacquiao, and added the WBA title with a win over Marcos Maidana two fights prior. Pacquiao had won titles in eight weight classes, amazingly retaining his power as he gradually moved up from light flyweight to welterweight. Two fights prior to meeting Mayweather, Pacquiao won the WBO title in a bout with Timothy Bradley.

While fans were not given the thriller the hype promised, they were treated to a Mayweather masterclass and a dogged effort from Pacquiao. For the entirety of the fight, Mayweather looked to use clean boxing while Pacquiao would burst with big flurries. Mayweather's skills led Pacquiao to throw significantly fewer punches than usual, a decrease in output that likely led to scores of 118-110, 116-112 and 116-112 all for Mayweather. But when both men earn more than $100 million, is either a loser?

Keith Thurman vs. Danny Garcia

An ugly build to the fight saw Danny Garcia's father hurl racial slurs at Keith Thurman during a pre-fight press conference. But the negativity couldn't overshadow what was to be a tremendous unification bout between undefeated champions. Thurman entered the fight 27-0 and WBA champ while Garcia was a sparkling 33-0 and WBC champ.

Adding to the hype for the rare welterweight unification clash was the fight's placement on CBS, where an average of 3.74 million viewers tuned in, the most for a prime-time boxing match since 1998.

Those who tuned in were treated to entertaining early rounds with both men slugging but Thurman pulling himself to an early lead. The action slowed midway through the bout as Thurman looked to nurse his lead on the scorecards to the final bell. This became almost a replay of De La Hoya vs. Trinidad as Thurman's caution and defensive adjustment led to him losing most of the late rounds on two of the three judges' scorecards, including one that ultimately tipped Garcia's way. Thurman was able to hold on for the split decision win, however, and unify the belts and make sure his was no the 0 to go.