During one of the many conflicts that lead up to Sweden’s secession from the Kalmar Union a successful invasion by Denmark into Sweden lead to what has become known as the Stockholm Bloodbath. 

After entering Stockholm with a mercenary army and the backing of the pope, Christian II of Denmark ordered the execution of over 80 Swedish nobles and clergymen. This took place over three days from November 7-9, 1520. The massacre was justified by Christian as necessary to maintain the union, but lead to only further outrage by an already alienated Swedish people. Two years later, lead by Gustav Vasa, the Danish army was finally driven out and the Kalmar Union dissolved.