The three most important political goals of King John were the restoration of the Kalmar Union, reduction of the dominance of the Hanseatic League, and the building of a strong Danish royal power.

Consequently, upon King Christian's death in May 1481, John's position was unchallenged in Denmark, whereas in Norway the Council of the Realm assumed royal authority, and an interregnum ensued.

By diplomatic means he tried to weaken the position of the Swedish regent Sten Sture, and he also sought new allies—he was the first Danish king to establish political cooperation with Russia.

At that time the position of the Hansabund was slowly but steadily declining because of changes in trade routes and the growing opposition against the Hanseatic League in the Northern European naval states.

In the meantime, John had initially approved to join a crusade plan as presented in Rome on 25 March 1490, in which Nordic soldiers would be part of a coalition of armies fighting the Turks of the Ottoman Empire; however, he sent a legate with a letter to Julius II to explain that he had many conflicts at home, which would prevent him from implementing such endeavor.

On 6 October 1497, John conquered Sweden during a short and effective military campaign, defeating Sten Sture at the Battle of Rotebro after having undermined his position by winning over most of the Swedish nobility.

In 1500, John made an attempt at conquering Dithmarschen (in today's Schleswig-Holstein), an area which the kings of Denmark had long viewed as belonging to their realm, but which was in reality an independent peasant republic under the loose overlordship of the Prince-Archbishopric of Bremen.

[11] By mid-January 1500 John hired the Black Guard of ruthless and violent Dutch and East Frisian mercenaries, commanded by Thomas Slentz, prior operating for Magnus, vicegerent in the Land of Hadeln.

[12][13] John guaranteed for its safe conduct first southeastwards via Lunenburg-Cellean Winsen upon Luhe and Hoopte, crossing the Elbe by Zollenspieker Ferry to the Hamburg-Lübeckian condominial Bergedorf and Vierlande.

John fought an increasingly more bitter war against Sten Sture and his successor Svante Nilsson, and this conflict meant frictions with both the Danish nobility and the Hanseatic cities, especially Lübeck.

Between 1510 and 1512, the king fought a last war with both Sweden and Lübeck in which Denmark was at first very pressed but, with the help of the Scottish Barton brothers,[15] partly turned the tables with a naval offensive.

Queen Christina, who lived the latter part of her life in a nunnery in Odense, commissioned the famous German sculptor Claus Berg to create a magnificent burial chapel, where both she and her husband were laid to rest after her death in 1521.

John's bloodline eventually returned to the Danish and Norwegian thrones in the person of Christian IV of Denmark, the great-great-grandson of his daughter, Electress Elisabeth.