Sweden and Norway had been united in 1319 under King Magnus Eriksson, and in 1397 Queen Margaret I of Denmark effects the personal union of Sweden, Norway, and Denmark through the Kalmar Union.

However, Margaret's successors, whose rule is also centered in Denmark, are unable to control the Swedish nobility.A large number of children inherit the Swedish crown over the course of the kingdom's existence; consequently real power is held for long periods by regents (notably those of the Sture family) chosen by the Swedish parliament.

King Christian II of Denmark, who asserts his claim to Sweden by force of arms, orders a massacre in 1520 of Swedish nobles in Stockholm.

This comes to be known as the "Stockholm blood bath" and stirs the Swedish nobility to new resistance and, on 6 June (now Sweden's national holiday) in 1523, they make Gustav Vasa their king.

This is sometimes considered as the foundation of modern Sweden.

Shortly afterwards he rejects Catholicism and leads Sweden into the Protestant Reformation.