On Tuesday, the Cavaliers dealt Kyrie Irving to the Celtics in a blockbuster deal that involved Isaiah Thomas. Per Elias Sports, it is the first offseason trade in NBA history with players who met in either the Conference Finals or NBA Finals. The Cavs eliminated the Celtics in five games in the Eastern Conference Finals last season, and now the rivalry gets a completely new dynamic with Irving and Thomas switching sides.
While a deal of this level is rare, it's certainly not the first time a player has joined a rival team, either via trade or free agency.
Ray Allen -- From the Celtics to the Heat, 2012
After winning a championship and making two Finals appearances with Boston, Allen rejected a contract offer worth $6 million per season and decided to sign for half of that amount to join LeBron James and the Miami Heat. His departure left Celtics teammates feeling very bitter. Kevin Garnett told reporters he didn't have Allen's number anymore. Earlier that year, Paul Pierce reflected on Allen's departure and said it rubbed him the wrong way that Allen didn't call fellow teammates to inform them of his decision. Allen ended up winning a second championship in 2013 and hit one of the biggest shots in Finals history.
Chris Chelios -- From the Blackhawks to the Red Wings, 1999
The rivalry between the Chicago Blackhawks and Detroit Red Wings -- two teams from the NHL's Original Six Era -- ran so deep that Chelios once said in a television interview as a member of the Blackhawks that he would never play for the Wings. After getting dealt at the 1999 Trade Deadline for Anders Eriksson and two first-round draft picks, the Hall-of-Fame defenseman admitted walking into the Detroit locker room felt like "crossing enemy lines." It was the first trade between the Red Wings and Blackhawks in 33 years. Chelios would go on to win two Stanley Cups in Detroit.
Brett Favre -- From the Packers to the Vikings, 2009 (after a season in N.Y.)
Favre should have retired a Green Bay Packer, but after the emergence of Aaron Rodgers, the Hall-of-Famer joined the New York Jets for a year, then signed with the Minnesota Vikings, a division rival of Green Bay. Favre led the Vikings to a 12-4 record and the NFC Championship in his first season in Minnesota, and appeared on the verge of a Super Bowl appearance that would have left long-time Packers fans feeling extremely conflicted, until this happened:
Bill Belichick -- From the Jets to the Patriots, 1999
After Bill Parcells stepped down as head coach of the New York Jets, the team had an arrangement in place in which assistant coach Belichick would succeed Parcells. Belichick's tenure as head coach of the Jets lasted one day. In what was supposed to be his introductory press conference, Belichick instead announced his resignation (via napkin: "I resign as HC of the NYJ"), and was named head coach of the rival New England Patriots instead. Five Super Bowl wins later, the rest, as they say, is history.
Kevin Durant -- From the Thunder to the Warriors, 2016
Durant was the most sought after free agent in the NBA last summer. After spending his first nine seasons with the Seattle Supersonics/Oklahoma City Thunder, and several playoff runs that came up short, no one would have faulted Durant for wanting to make a run at the title elsewhere, but few thought he would choose the Golden State Warriors, the team that had just come back from a 3-1 deficit in the Western Conference Finals to defeat OKC. But that's exactly happened, and the Warriors not only became a super team, but also eliminated their top rival in the West from title contention. In his first season, Durant won his first NBA title and was named Finals MVP.
Deion Sanders -- From the Niners to the Cowboys, 1995
After winning a Super Bowl with the San Francisco 49ers in 1994 -- in which he won Defensive Player of the Year and had an interception in the team's Super Bowl win over the San Diego Chargers -- Sanders signed with the rival Dallas Cowboys, a team he had tormented throughout his career. In Sanders' first season in Dallas, the Cowboys won their third Super Bowl in four seasons.
Vince Carter -- From the Raptors to the Nets, 2004
Carter put the Toronto Raptors on the national stage, and left such a significant impact on basketball in Canada there's an entire documentary devoted to him being released next month at the Toronto International Film Festival. From the legendary 2000 Slam Dunk contest appearance to his playoff duel with Allen Iverson, Carter won over the hearts of fans in Toronto. But by 2004, Carter's frustration with the franchise's inability to build a contender reached a boiling point, and after demanding a trade, he was dealt within the division to the New Jersey Nets in December for Alonzo Mourning, Eric Williams, Aaron Williams and two first-round draft picks. For years, Raptors fans booed Carter whenever he returned, but during their 20th anniversary season in 2014, fans greeted their former star with a standing ovation after a video tribute.
Dennis Rodman -- From the Pistons to the Bulls, 1995
As a member of the "Bad Boy" Pistons, Rodman was a rival of Michael Jordan and Scottie Pippen. Given all of their physical battles in the playoffs, it was hard to imagine the three of them ever being teammates, let alone winning three championships together. But that's exactly what happened. Before the 1995-96 season, the Bulls traded for Rodman (giving up center Will Perdue and cash), believing Jordan, Pippen and Phil Jackson could manage the volatile personality. In the end, they did enough to keep the rebounding king focused on his task. The Bulls went three straight titles from 1996-1998.
Babe Ruth -- From the Red Sox to the Yankees
No rival transaction will probably ever top the Yankees purchasing the contract of Ruth from the Boston Red Sox in 1920 for $125,000 to help Sox owner Harry Frazee finance his ventures in musical theater. The Curse of the Bambino lasted 86 years, and any mention of players switching teams to a rival and coming back to haunt them will always begin and end with the Babe.