Winning the Treble is undoubtedly one of the most impressive feats of modern football.
Winning the league title is an incredible achievement that shows the endurance and stability of a team.
Winning the Champions League or domestic cup shows the mental strength of the team and their ability to perform in life-and-death moments.
Even winning two of these three trophies is an extraordinary achievement for any club.
But winning these three titles in one season is indeed a historic achievement of club football. United in 1998–99 was the first team to win a treble since the 1992 Champions League reform.
Since then, only three clubs have achieved this feat, two of which have succeeded twice, and let's take a look at the five teams that have won the Treble in the 21st century.
5. Inter Milan (2009-10 season)
Just over a decade ago, Inter experienced one of the most historic seasons in the history of Italian football. Under Mourinho, the Nerazzurri achieved achievements that no other Italian team had ever achieved.
The Serie A giants have won Serie A, the Coppa Italia and the UEFA Champions League, becoming the first and only Italian team to achieve a triple crown to date.
Mourinho took over the team in 2008 and led the team to a Title One title in his first season, which was not enough for Inter fans, as the club dominated Serie A at the time but made no progress in the Champions League.
So the following summer, Mourinho disbanded the team and began to form his own army. Figo retired and Adriano, Cruz and Crespo all left the club and were soon replaced by Motta, Diego Milito and Sneijder.
Even Ibrahimovic left Inter as a player plus cash, while Barcelona sent Eto'o in a different way.
Mourinho has built an experienced and strong-willed team, but it is also full of star talent and the ability to win games. Inter's formidable counter-attacking tactics, built on a strong defensive foundation, set off a storm in Serie A.
They narrowly beat Roma by two points to win the Pride A title for the second consecutive season. New goalscorer Milito had a strong debut season at the club, with the Argentine scorer scoring 22 goals, including some key ones.
Inter milan beat Roma in the Coppa Italia final with a goal from Milito to win the domestic league double.
In the Champions League, Inter beat Jose Mourinho's old club Chelsea in the round of 16 and then easily defeated CSKA Moscow, winning by a small one in both legs, before meeting Cosmos Barca.
It appears Mourinho has learned his lesson from his earlier defeat to Barcelona, when Inter won 3-1 in Meazza. The second leg was one of the most exciting and memorable moments in the history of the Champions League.
Inter conceded just one goal at Camp Nou in a 10-man battle and beat Barcelona on aggregate to reach the final. Or Milito's two goals, Inter won 2-0 in the final against Bayern Munich, and Inter's name is recorded in football history.
4. Bayern Munich (2012-13)
Bayern Munich had a disappointing ending in 2011-12, with Bayern losing to Borussia Dortmund in the Bundesliga for second place and losing to Chelsea in the Champions League final.
However, in the following season, Heynckes's Bayern lost only one game in the Bundesliga and regained the throne of the new title.
They ended the season with 91 points, leading second-placed Dortmund with a staggering 25 points, with the two teams meeting in the final of the German Cup, with goals from Mandzukic and Thomas Müller helping Bayern beat Borussia Dortmund to complete the domestic double-title mission.
The two sides met again, this time in the Champions League final at Wembley in May 2013. Mandzukic once again opened the scoring for Bayern, but this time Borussia Dortmund equalised through Gundogan.
However, Borussia Dortmund's heart will be shattered in the final moments of the game. Winger Robben showed a brilliant goal to win the title for Bayern and complete the first treble in the club's history.
3. Barcelona (2014-15 season)
After losing La Liga in 2013-14, Barcelona made a series of changes, appointing Enrique as manager and signing Suarez from Liverpool.
Under Enrique, Barca made a brilliant start, with Barcelona losing just three games before the end of the season to win the La Liga title. Despite scoring 94 points this season, they won the title by just two points, with Real Madrid trailing all the way to the end.
Just a few days after winning La Liga, Barcelona finished with an unforgettable league double by beating Athletic Bilbao 3-1 in the Copa del Rey final.
In the Champions League, Barcelona beat Manchester City, Paris Saint-Germain and Bayern Munich to hold the final with Juventus in Berlin.
In the match against the Bianconeri, Barcelona played at their best level, winning the race 3-1 and ending a perfect season.
The trident of Messi, Suarez and Neymar scored a staggering 122 goals this season, playing a major role in the second treble in the club's history.
2. Bayern Munich (2019-20)
Bayern Munich had a bland start to the 2019-20 season under Kovac, but after Frick replaced him in the middle of the season, Bayern looked unstoppable.
Not only has Frick stabilized the sinking Bavarian battleship, but it has also established a perfect balance between young players and veterans.
Although Bayern didn't lead the Bundesliga until their 20th matchday in early 2020, they never stopped once they got there.
Their last defeat came against Borussia Borussia Monchengin in early December, and after that defeat Bayern went unbeaten in 29 games and ended the season with 21 consecutive wins.
Bayern also became the first team to win every game in the Champions League, scoring an incredible 41 goals over the course of the game, including historic wins over Tottenham Hotspur (7-2), Red Star Belgrade (6-0) and Barcelona (8-2).
Despite their difficult 1-0 win over Paris Saint-Germain in the final, for Bayern they won their second treble in history.
It was also an unforgettable season for their striker Lewandowski, who scored 55 goals in all competitions that season.
Bayern then won the European Super Cup, the German Super Cup and the Club World Cup, becoming the second team in history to win six titles.
1. Barcelona (2008-09 season)
Barcelona entered a new era of domination this season, which put Guardiola and Messi in the spotlight of football, and Barcelona's 2008-09 season was undoubtedly the best team of the 21st century.
After two seasons under Rijkaarde without winning the title, Barcelona decided to appoint Guardiola as a first-team coach in 2008.
Guardiola made some big changes to his team, even selling what he thought was redundant with Ronaldinho and Deco. By the end of the season, Barca had won 27 of 38 league games, including the famous 6-2 away win over Real Madrid.
Just a few days before the final league game of the season, Barcelona beat Athletic Bilbao 4-1 in the Copa del Rey final. This completed a historic double for Barca in guardiola's first season in charge.
Barcelona beat Lyon, Bayern Munich and beat Chelsea in the semi-finals to reach the Champions League final. Guardiola's side faced a tough test in the final as United won the Champions League in 2008 and they hope to defend their title.
However, thanks to Messi's outstanding performances, Barcelona completed their historic treble with a 2-0 victory over Manchester United in Roma, and they became the first Spanish team to win the treble.
At the same time, Barça also won the European Super Cup, the Spanish Super Cup and the Club World Cup, becoming the first team to win six titles.
It was indeed the beginning of Barcelona's glory days, and their team, considered by many at the time to be the best in the history of football, was known as the Cosmic Team.