Before becoming king, Harald spent 15 years in exile as a mercenary and military commander in Kievan Rus' and chief of the Varangian Guard in the Byzantine Empire.

[16] The Icelandic sagas, in particular Snorri Sturluson in Heimskringla, claim that Sigurd, like Olaf's father, was a great-grandson of King Harald Fairhair in the male line.

After a friendly welcome, Olaf went on to gather an army and eventually fight in the Battle of Stiklestad on 29 July 1030, in which Harald took part on his brother's side.

[20] After the defeat at the Battle of Stiklestad, Harald managed to escape with the aid of Rögnvald Brusason (later Earl of Orkney) to a remote farm in Eastern Norway.

[25] Harald took part in Yaroslav's campaign against the Poles in 1031, and possibly also fought against other 1030s Kievan enemies and rivals such as the Chudes in Estonia, and the Byzantines, as well as the Pechenegs and other steppe nomad people.

By 1035, the Byzantines had pushed the Arabs out of Asia Minor to the east and southeast, and Harald took part in campaigns that went as far east as the Tigris River and Euphrates River in Mesopotamia, where according to his skald (poet) Þjóðólfr Arnórsson (recounted in the sagas) he participated in the capture of eighty Arab strongholds, a number which historians Sigfus Blöndal and Benedikt Benedikz see no particular reason to question.

Modern historians have speculated that Harald may have been in a party sent to escort pilgrims to Jerusalem (possibly including members of the Imperial family) following the peace agreement, as it was also agreed that the Byzantines were allowed to repair the Church of the Holy Sepulchre.

[36] In 1041, when the Byzantine expedition to Sicily was over, a Lombard-Norman revolt erupted in southern Italy, and Harald led the Varangian Guard in multiple battles.

[31] There, he fought in the army of Emperor Michael IV in the Battle of Ostrovo of the 1041 campaign against the Bulgarian uprising led by Peter Delyan, which later gained Harald the nickname the "Bulgar-burner" (Bolgara brennir) by his skald.

[58] During his second stay there, he married Elisabeth (referred to in Scandinavian sources as Ellisif), daughter of Yaroslav the Wise and granddaughter of the Swedish king Olof Skötkonung.

[60] Seeking to regain for himself the kingdom lost by his half-brother Olaf Haraldsson,[54] Harald began his journey westwards in early 1045, departing from Novgorod (Holmgard) to Staraya Ladoga (Aldeigjuborg) where he obtained a ship.

[77] Although Magnus had appointed Sweyn his successor as king of Denmark, Harald immediately announced his plans to gather an army and oust his former ally from the country.

[80] Harald also wanted to re-establish Magnus's rule over Denmark,[15] and in the long term probably sought to restore Cnut the Great's "North Sea Empire" in its entirety.

[83] One of two conventional battles was set to be fought between the two kings later the same year, but, according to Saxo Grammaticus, Sweyn's smaller army was so frightened when approached by the Norwegians that they chose to jump in the water trying to escape; most drowned.

Harald's contribution to the strengthening of Norway's monarchy was the enforcement of a policy that only the king could retain a hird, thus centralising power away from local warlords.

[93] The primary opposition to Harald's rule would be the descendants of Haakon Sigurdsson, from the powerful dynasty of Earls of Lade who had controlled Northern Norway and Trøndelag with much autonomy under the Norwegian king.

Although the sources differ on the circumstances, the next event nonetheless led to the murder of Einar by Harald's men, which threatened to throw Norway into a state of civil war.

Although the remaining descendants of Haakon Sigurdsson considered rebellion against the king, Harald eventually managed to negotiate peace with them, and secured the family's submission for the remainder of his reign.

Harald is considered to have instituted good economic policies, as he developed a Norwegian currency and a viable coin economy, which in turn allowed Norway to participate in international trade.

Who, having searched thoroughly the length of the northern ocean in ships, finally had before his eyes the dark failing boundaries of the savage world, and, by retracing his steps, with difficulty barely escaped the deep abyss in safety.Kelly DeVries has suggested that Harald "may even have known of and sought out the legendary land called Vinland, which Viking sailors had discovered only a short time before", which Adam mentions earlier in the same passage to have been widely reported in Denmark and Norway.

[107] Accepting he could not conquer Denmark, Harald switched attention to England; his claim was based on a 1038 agreement between Magnus and its previous ruler, Harthacnut, who died childless in 1042.

[111] Harold's brother Tostig Godwinson, formerly Earl of Northumbria, now appeared on the scene; hoping to regain his titles and lands, he reportedly approached both William and Sweyn Estridsson for their support.

[113] In March or April 1066, Harald began assembling his fleet at Solund, in the Sognefjord, a process completed by the start of September 1066;[114] it included his flagship, Ormen, or "Serpent".

[119] The chronicler, John of Worcester, suggests he left Flanders in May or June, raiding the heartland of Harold's estates in southern England, from the Isle of Wight to Sandwich.

News of the early raids had reached the earls Morcar of Northumbria and Edwin of Mercia, and they fought against Harald's invading army three kilometres (2 mi) south of York at the Battle of Fulford, also on 20 September.

[124] The same day as York surrendered to Harald and Tostig, Harold Godwinson arrived with his army in Tadcaster, just eleven kilometres (7 mi) from the anchored Norwegian fleet at Riccall.

They brought only light armour, as they expected to just meet the citizens of York, as they had agreed the day before, at Stamford Bridge to decide on who should manage the town under Harald.

Most of the forces from Scotland and Orkney probably remained at Riccall throughout the battle (the earls Paul and Erlend Thorfinnsson are certainly known to have been stationed there the entire time), and has not been counted in the traditional figure.

The fact that Harold had to make a forced march to fight Hardrada at Stamford Bridge and then move at utmost speed south to meet the Norman invasion, all in less than three weeks, is widely seen as a primary factor in William's victory at Hastings.

[139] According to one poem, Harald had mastered a number of activities that were considered sports in the Viking Age, in addition to poetry, brewing, horse riding, swimming, skiing, shooting, rowing and playing the harp.