How the rivalry developed
Der Klassiker is a modern invention. The clubs are around 600 kilometres apart and share no local history, and the name was coined for international audiences in the 2010s — which is precisely when the fixture started deciding titles.
The heat came from three things happening at once. Dortmund, under Jürgen Klopp, won the Bundesliga in 2011 and 2012 and beat Bayern 5–2 in the 2012 cup final. Bayern responded by winning the treble in 2013 and by signing Dortmund's players: Mario Götze in 2013, Mats Hummels in 2016, and Robert Lewandowski on a free transfer in 2014. Each deal was legal, well-timed and deeply resented.
The result is a rivalry that runs one way emotionally. For Dortmund it is the defining fixture; for Bayern it is the biggest of many. That asymmetry is the most misunderstood thing about it in English coverage.
The matches that defined it
The 11–1 in November 1971 remains Bayern's record Bundesliga win and Dortmund's heaviest defeat. The 2012 cup final, won 5–2 by Dortmund, was the high point of Klopp's side. Then came Wembley in 2013: the first all-German European Cup final, settled 2–1 by an Arjen Robben goal in the 89th minute, with Bayern completing the treble days later.