The two reigned over Sweden together until 1364, when they were deposed in favor of Magnus' sororal nephew, Albert of Mecklenburg, by a clique of exiled Swedish noblemen led by Bo Jonsson Grip.

[2][3][9] Barely a year later, representatives of the cities and the general public assembled at Båhus Castle, where they hailed Haakon as their king and took the oath of perpetual fealty and servitude to him.

[2][3][9] The first documented event in which Haakon acted as sole king and ruler over his kingdom was on 22 January 1358, when he sent a letter of approval for the privileges in the city of Oslo.

[2][11] As king, Haakon was immediately pulled into his father's internal strife in Sweden, where a growing conflict eventually had erupted into open warfare.

Eric was discontent with his father's rule, likely because he had not been granted a membership in the Swedish Council of the Realm, and because of favouritism Magnus had shown his youngest son.

In accordance with the peace agreement between father and eldest son in 1357, the Swedish nobility soon deposed Magnus and elected Haakon as King of Sweden in Uppsala 15 February 1362.

When the castle was captured, Valdemar had virtually regained control over all of Scania, and when Magnus proved to be unable to retake the province by force, it simply passed back to Danish rule.

[2][24] In 1364, Duke Albert enlisted military support from several powerful North German noblemen and proceeded to swiftly invade and conquer Sweden and subsequently installed his son as the new king.

On 27 February, Haakon issued a proclamation against Albert of Mecklenburg and his supporters, encouraging the local populace to stir up in rebellion against the German usurper.

He entered into an alliance with King Valdemar, the father of his wife, which was something that later would dramatically concentrate the Norwegian foreign policy to the east, rather than to the traditional west.

[3][29][30][32] In 1361, Valdemar had invaded and conquered the Swedish province of Scania, as well as the two islands of Öland and Gotland, and captured the major Hanseatic town of Visby in the process.

[2] Valdemar was unable to enforce the fragile peace with the Hanseatic cities, and in 1367 the League founded the Confederation of Cologne against Denmark and Norway to counter the growing ambitions of the two allied kings.

The Confederation renewed their alliance with German-held Sweden and assembled a large fleet of warships and subsequently assaulted the Norwegian coast and continued to raid it all the way to Agder.

The Confederation raided and pillaged the Danish coast, invaded the province of Danish-held Scania, and even captured and looted the city of Copenhagen through a successful siege.

Realizing the futility in a prolonged and costly war, as well as Haakon's wavering support for the disastrous conflict, Valdemar appointed his friend and advisor, Rigsdrost Henning Podebusk, to negotiate peace with the Confederation in his absence.

In addition to this, the Confederation forced Valdemar to grant the Hanseatic League a considerable amount of influence over the future Danish king-elections, including the right to veto against any throne candidate.

[33] Lastly, the Confederation put several towns on the coast of Scania and Helsingborg Castle under the control of the Hanseatic League for a fixed period of fifteen years.

[34][35][36][37][38] Haakon became less interested in Danish affairs following the military failures against the Hanseatic cities, and would again turn his attention to his chief foreign policy of retaking Sweden from the Germans, something which he would have some limited success with in the following years.

However, Haakon would gain a renewed interest in Danish affairs when the opportunity to have his son elected king of Denmark arose upon Valdemar's death in 1375.

[42][43] Haakon, having already lost a substantial portion of his Swedish realm to the Germans, would go to great and costly lengths to assure his son's accession to the Danish throne.

[2][44] In 1349, the Black Death was brought to Bergen on board an English merchant ship to devastating effects, killing between 50% and 60% of the population,[45] leaving the kingdom in a period of social and economic decline.

[9][46][47][48] In 1371, Haakon acquired the release of his father from German imprisonment against a large ransom sum of 12,000 silver marks, which alone was more than the kingdom's ordinary tax incomes just before the outbreak of the plague.

In return, Sinclair pledged to pay a sum of 1000 nobles before 11 November the same year, and when called upon, was to serve the Norwegian king on Orkney or elsewhere with 100 equipped and armed men for up to three months.

Also, throughout his rule, especially after the loss of Sweden to the Germans, a noticeable number of Swedish noblemen pledged their support to Haakon and settled on Norwegian lands.

[49][52] Haakon never ceased to pursue his inherited responsibility to reclaim the lost Swedish territories, and would in March 1380 issue letters to prepare for war against the Germans in Sweden.