Real Madrid will make its second appearance in Philadelphia after defeating the Union last year, 2-1, in front of 57,312 at Lincoln Financial Field.
Real Madrid, founded in 1902, returns three months after supplanting FC Barcelona as champion of Spain's La Liga. Madrid has earned that title 32 times and won the Champions League nine times - in both cases, more than any other club.
Throughout its history, Real Madrid relied on a bevy of international stars such as David Beckham, who played from 2003 to 2007 before joining the Los Angeles Galaxy. Madrid secured the 2006-07 La Liga title in Beckham's last game.
Beckham's teammates included France's Zinedine Zidane; Portugal's Luis Figo; and Brazil's Roberto Carlos, Ronaldo, and Raul, the club's all-time leading scorer.
"They've always had the best players in the world there," Beckham said. "Just being part of such an historic club, to play with all these great players and then win a championship, that was obviously a special time for me."
Among the current players is Ronaldo, who finished second to Barcelona's Lionel Messi in voting for FIFA's 2012 Ballon d'Or, awarded to the world's best player.
Casillas, the goalkeeper and captain, helped Spain win the 2010 World Cup. He has played more games for Spain than anyone else and owns a record of 509 consecutive scoreless minutes in the European championships.
Joining Casillas on the World Cup winners and this year's European champions were four of his Real Madrid teammates: midfielder Xabi Alonzo, defenders Sergio Ramos and Alvaro Arbeola, and reserve goalkeeper Raul Albiol.
Ozil, 23, is a playmaking midfielder who led La Liga with 17 assists last season and helped Germany reach the World Cup semifinals two years ago. Ozil also scored one of Real Madrid's two goals in last year's victory against the Union.
Coaching the team is Jose Mourinho, who won seven league championships in four countries and the Champions League twice. Last season, Mourinho's squad set a European record for points (100) and La Liga standards for victories (32) and goals (121).
"Mourinho has made me a better player," said Arbeola, a starter in the 2010 World Cup. "We have a coach who always demands 100 percent."
As usual, this year's squad faces great expectations.
"There has always been a lot of pressure on this team, and there always will be," Arbeola said. "Each year, we keep aiming to win titles. The more, the merrier."
Deal is done. On Friday at the Hyatt Regency in Philadelphia, Celtic FC management and Philadelphia Sports Zone founders announced a partnership, with Celtic FC bringing its International Development Academy to Philadelphia. The location of the academy has yet to be announced.