Christian I was a Scandinavian monarch under the Kalmar Union. He was King of Denmark (1448–1481), Norway (1450–1481) and Sweden (1457–1464). From 1460 to 1481, he was also Duke of Schleswig (within Denmark) and Count (after 1474, Duke) of Holstein (within the Holy Roman Empire). He was the first Danish monarch of the House of Oldenburg.
He was a son of Count Dietrich of Oldenburg, a descendant of King Eric IV of Denmark, and Hedwig of Holstein, a descendant of King Eric V of Denmark and Abel of Denmark. In the power vacuum that arose following the childless death of King Christopher of Denmark, Sweden and Norway in 1448, Sweden elected Charles VIII king with the intent to reestablish the union under a Swedish king. Charles was elected king of Norway in the following year, but the counts of Holstein were more influential than the Swedes and the Norwegians together, and made the Danish Privy Council appoint Christian as king. His subsequent accessions to the thrones of Norway (in 1450) and Sweden (in 1457), restored the unity of the Kalmar Union for a short period. In 1460, following the childless death of his uncle, Duke Adolphus of Schleswig, Count of Holstein, Christian also became Duke of Schleswig and Count of Holstein.
In 1463, Sweden broke away from the union for the first time, and Christian's attempt at a reconquest resulted in his defeat to the Swedish regent Sten Sture the Elder at the Battle of Brunkeberg in 1471.
In the 1470s, he undertook several costly travels, in 1474 to Pope Sixtus IV, during which he obtained permission to found the University of Copenhagen which opened in 1479.
On 20 June 1448 Sweden elected Charles Knutsson as king. Norway was now faced with the choice between a union with Denmark or Sweden, or electing a separate king. The latter option was quickly discarded, and a power-struggle ensued between the supporters of Christian of Denmark and Charles of Sweden. The Norwegian Council of the Realm was divided. In February 1449, a part of the Council declared in favour of Charles as king, but on 15 June the same year, a different group of councillors paid homage to Christian. On 20 November, Charles was crowned king of Norway in Trondheim. However, the Swedish nobility now took steps to avoid war with Denmark. In June 1450, the Swedish Council of the Realm forced Charles to renounce his claim on Norway to king Christian. The question of the Norwegian succession had thereby been decided between Denmark and Sweden, and the Norwegian Council was left with only one candidate for the throne. In the summer of 1450, Christian sailed to Norway with a large fleet, and on 2 August he was crowned king of Norway in Trondheim. On 29 August, a union treaty between Denmark and Norway was signed in Bergen. Norway had of old been a hereditary monarchy, but this had become less and less a reality, as at the last royal successions, hereditary claims had been bypassed for political reasons. It was now explicitly stated that Norway, as well as Denmark, was an elective monarchy. The treaty stipulated that Denmark and Norway should have the same king in perpetuity, and that he would be elected among the legitimate sons of the previous king, if such existed.
Charles Knutsson became increasingly unpopular as king of Sweden, and was driven into exile in 1457. Christian achieved his aim of being elected as king of Sweden, thus re-establishing the Kalmar Union. He received the power from temporary Swedish regents archbishop Jöns Bengtsson Oxenstierna and lord Erik Axelsson Tott. However, Sweden being volatile and split by factions (benefits of union being against nationalistic benefits), his reign there ended in 1464 when bishop Kettil Karlsson Vasa was installed as the next regent. Charles Knutsson was recalled as King of Sweden, although he was later exiled a second time, recalled again and died during his third term as king. Christian's final attempt at regaining Sweden ended in a total military failure at Brunkeberg (outside Stockholm) October 1471 where he was defeated by the Swedish regent Sten Sture the Elder who was supported by the Danish-Swedish nobleman's clan the Thott family. Christian maintained his claim to the Swedish kingdom up to his death in 1481.
Christian's personal territory was at its largest in 1460–64, before the loss of Sweden. However, many parts of his realm wanted to govern themselves locally, and there were constant struggles. Denmark was his most important center of power. In 1474 Christian travelled two times: in April he went to Milan (his stay in Lombardy is celebrated by frescoes by Il Romanino in the Malpaga Castle) and Rome, in Italy, where he met Pope Sixtus IV. In that occasion, his wife received by the pope the authorization to found a university in Copenhagen, which would be created in 1478 or 1479. In the autumn same year he visited Charles of Burgundy, acting as intermediary between him and future emperor Maximilian I. He stayed in Burgundy for several months, moving to the Netherlands in the early 1475.
King Christian died at Copenhagen Castle on 21 May 1481, at the age of 55.