Mats Hummels has spoken about why he is crossing one of German soccer’s biggest divides.

The 2014 FIFA World Cup-winning center back revealed Tuesday his reason for deciding to leave Borussia Dortmund and join Bayern Munich this summer. Hummels, 27, released a statement on Facebook in which called his impending transfer “the most difficult the decision of my life,” according to German broadcaster Deutsche Welle

Nachdem heute mein Transfer zum FC Bayern bekannt gegeben wurde, möchte ich mich nun auch noch etwas ausführlicher dazu…

Posted by Mats Hummels on Tuesday, May 10, 2016

Bayern Munich CEO Karl-Heinz Rummenigge is elated to strengthen his team at a rival’s expense.

“Mats is one of the best central defenders in the world,” Rummenigge said, according to The BBC. “With him, we can increase the quality of our team again.”

Baeyern Munich, winner of the last four Bundesliga (German first division) titles and the 2013 UEFA Champions League, announced on Tuesday that Hummels had signed a five-year contract with the club. Neither Bayern Munich nor Borussia Dortmund disclosed the transfer fee, but German newspaper Bild reports it’s jaw-dropping in Bundesliga terms.

Hummels returns to his boyhood club after eight outstanding seasons with Borussia Dortmund. His last game with Dortmund is likely to be the DFB-Pokal (German Cup) final on May 21 against Bayern Munich.

Bayern Munich announced Hummels’ signing just 30 minutes after it revealed its £27.5 million (€35 million/$40 million) acquisition of 18-year-old midfielder Renato Sanches from Benfica.

Hummels and Sanches will work under the watchful eye of Carlo Ancelotti, who will succeed Pep Guardiola as Bayern Munich’s head coach in July.

Bayern Munich’s rivals at home and abroad should be afraid. Be very afraid.

Thumbnail photo via Tim Groothuis/Witters Sport via USA TODAY Sports Images

Thumbnail photo via July 13, 2014; Rio de Janeiro, BRAZIL; Germany defender Mats Hummels (5) heads a ball against the Argentina in the championship match of the 2014 World Cup at Maracana Stadium. Mandatory Credit: Tim Groothuis/Witters Sport via USA TODAY Sports