Legally, the countries remained separate sovereign states, but their domestic and foreign policies were directed by a common monarch.

The main reason for its failure to survive was the perpetual struggle between the monarch, who wanted a strong unified state, and the Swedish and Danish nobility, which did not.

[8] The Union lost territory when Orkney and Shetland were pledged by Christian I, in his capacity as King of Norway, as security against the payment of the dowry of his daughter Margaret, betrothed to James III of Scotland in 1468.

[10] Diverging interests (especially the Swedish nobility's dissatisfaction with the dominant role played by Denmark and Holstein) gave rise to a conflict that hampered the union in several intervals starting in the 1430s.

[11] King Erik's foreign policy, in particular his conflict with the Hanseatic League, necessitated greater taxation and complicated exports of iron, which in turn may have precipitated the rebellion.

[11] After Karl's death, Sweden was mostly ruled by a series of "protectors of the realm" (riksföreståndare), with the Danish kings attempting to assert control.

[11] Sten Sture the Younger was killed in the 1520 Battle of Bogesund when the Danish king Christian II invaded Sweden with a large army.

[11] Subsequently, Christian II was crowned King of Sweden, and supporters of Sten Sture were executed en masse in the Stockholm Bloodbath.

[11] After the Stockholm Bloodbath, Gustav Vasa (whose father, Erik Johansson, was executed) travelled to Dalarna, where he organized a rebellion against Christian II.