To understand the depths of animosity between the Miami Heat and Boston Celtics entering this year's NBA Eastern Conference finals, you have to go back to an otherwise nondescript regular-season night in March 2013.

That night in New Orleans, a Heat public relations official approached the team's traveling media contingent with a message from team president Pat Riley that would immediately go into the annals of history.

"Danny Ainge needs to shut the f--- up and manage his own team," Riley said, via that hand-delivered statement. "He was the biggest whiner going when he was playing and I know that because I coached against him."

For Riley, of course, the animosity wasn't just about Ainge criticizing then-Heat star LeBron James and his tendency to complain about officiating. Riley's beef with Ainge, and the entire Celtics organization, was personal.

While he has spent nearly three decades as the godfather of Miami Heat basketball, lording over one of the league's most successful and stable franchises, Riley cut his teeth as a player and coach with the Los Angeles Lakers, and spent many years duking it out for championships with their rivals in Boston.

As Riley's Heat have consistently stationed themselves among the East's elite over the past two decades, the team most often pitted against them has been the Celtics. As Miami makes its eighth trip to the East finals over the past 18 years and Boston makes its seventh when the two teams square off in Game 1 on Tuesday at FTX Arena (8:30 p.m. ET on ESPN), that shared history helps inform what could lie ahead.

Everything ties back to the summer of 2007, when the Celtics traded for Kevin Garnett and Ray Allen to play alongside Paul Pierce. Boston won the 2008 NBA title and returned to the NBA Finals two years later. But that's only part of the story. The Celtics' star-powered ascent to the top of the East played a pivotal role in James and Chris Bosh choosing to take their talents to South Beach to join Dwyane Wade there in the summer of 2010.

Boston took down James and the Cleveland Cavaliers in the second round in both 2008 and 2010. Pierce outdueled James in Game 7 in 2008, and then caused James to have one of his two worst playoff series of his career -- along with the 2011 NBA Finals -- to stun the top-seeded Cavaliers in 2010. One round earlier, Pierce hit a buzzer-beater in Game 3 to all but send the Heat home for the summer.

Those collective failures helped lead to "The Decision" and the three young stars in their prime deciding they had to team up.

Then, it was time for the Heat to return the favor -- first by smashing the Celtics in five games in the second round in 2011, closing out Game 5 with a 16-0 run to officially vanquish their rivals. It was the first time James or Wade had beaten Boston in the postseason.

Things were cranked up a notch the following season, with the teams meeting in the conference finals. After the Heat took a 2-0 lead, Boston ripped off three consecutive wins, pushing Miami to the brink of elimination. Following the Heat's flameout in the NBA Finals the year before, Miami's Game 5 loss -- punctuated by a fan telling the defeated Heat "good job, good effort" as they left the floor -- had critics suggesting it was time for Miami's big three to split up.

All that did, however, was set the stage for what became the signature moment in James' career: his 45-point, 15-rebound game that helped Miami win in Boston, allowing the Heat to come from behind in the series and, a couple of weeks later, go on to win the title -- the first in James' career.

"In an environment like this, you want to have a big game," James said that night.

Those East finals 10 years ago proved to be the last gasp of greatness for the Pierce-Garnett-Allen triumvirate. But the final blow came a couple weeks later, when Allen shocked the basketball world and chose to join -- of all teams -- the Heat in free agency, a move that set the stage for a decade of animosity between the former teammates.

And while Allen went on to hit one of the greatest shots in NBA history the following year to help deliver Miami a second straight title, Boston laid the foundation for its current roster -- by tearing it down.

In a seismic transaction, the Celtics sent Pierce, Garnett and Jason Terry to the Brooklyn Nets for a bevy of draft picks -- two of which eventually turned into Jayson Tatum and Jaylen Brown, the one-two punch now leading the Celtics four wins from the Finals a decade later.

Across the intervening years, the two teams mostly missed each other; Miami made the NBA Finals in 2013 and 2014; Boston made the Eastern finals in 2017 and 2018.

But then came the NBA's bubble at the Walt Disney World Resort two seasons ago, when these two teams again squared off. Bam Adebayo snatched Game 1 away from Boston with an all-timer of a block of Tatum at the rim in the final seconds of overtime, setting the stage for Miami's eventual victory in six games in a series in which Miami imposed its will on Boston.

"They were just more aggressive," Celtics guard Marcus Smart said after Boston saw its season end to the Heat. "They were getting whatever they wanted. ... Unfortunately, we didn't combat it. We didn't respond the way we should have.

"It's part of it. We played a really good team. Gotta tip your hat off to those guys. Gotta go back to the drawing board and see what needs to be fixed and come back ready next year."

It took two years, but "next year" is here. Once again, we've seen one team respond to what the other did. After Miami pounded Boston into the ground across those East finals, the Celtics this season have turned into a team that prides itself on aggressive, physical play.

First-year Celtics coach Ime Udoka constantly uses the word "physicality" to describe his team's play. Boston has size across the board, and uses it to wear down its opponents, as it did to both the Brooklyn Nets in the first round and the Milwaukee Bucks in the second.

Ainge has since left, and many of the other ties to those clashes from 10 years ago are gone from Boston, but in Miami, those memories run deep. The Heat organization is full of people who were around for every one of those previous memorable battles. That group is, of course, led by Riley, but also includes coach Erik Spoelstra, veteran forward Udonis Haslem and scores more.

"You have two really committed organizations and teams to do things at a high level on both ends of the floor," Spoelstra said. "You have teams that have built up a lot of habits over the months of the regular season. It's not going to be anything over the top. It's just going to be really good competition."

Sounds like a perfect time for yet another playoff clash between the Celtics and Heat.