2015 would mark the first time in Major League history that the Cardinals and Cubs met in the postseason. Chicago had defeated the Pirates in the Wild Card game to advance to the NLDS.
The Cardinals had the best record in baseball, but the NL Central was a three-way battle. St. Louis won 100 games, but the third-place Cubs had won 97, so the NLDS was sure to be a fun battle.
Things started off well, as the Cardinals won Game 1 thanks to a dominant performance by John Lackey. But things went south from there. After losing Game 2 at home, the Cardinals embarked on a trip to Chicago, where they would drop the final two games in heartbreaking fashion, losing to their hated rivals and watching them go to the NLCS.
It was a rare moment where the Cubs had bragging rights over the Cardinals. This officially marked a paradigm shift in the Cubs-Cards rivalry. The Cardinals would not reach the postseason again until 2019, while the Cubs would win their first World Series title since 1908 the following year.
To make matters worse, Lackey and Jason Heyward left the Cardinals at the end of the 2015 season and were key factors in the Cubs' run to the title the following year.