He unsuccessfully claimed the Danish throne until and the English throne in 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. In his chronicle , Adam of Bremen called him the " Thunderbolt of the North ". In , the fifteen-year-old Harald fought in the Battle of Stiklestad with his half-brother Olaf Haraldsson. Olaf sought to reclaim the Norwegian throne, which he had lost to Danish king Cnut two years prior.
Olaf and Harald were defeated by forces loyal to Cnut, and Harald was forced into exile to Kievan Rus'. Thereafter, he was in the army of Grand Prince Yaroslav the Wise , becoming captain, until he moved on to Constantinople with his companions around In Constantinople, he rose quickly to become the commander of the Byzantine Varangian Guard, seeing action on the Mediterranean Sea , in Asia Minor , Sicily, possibly in the Holy Land , Bulgaria and in Constantinople itself, where he became involved in the imperial dynastic disputes.
Harald amassed wealth whilst in the Byzantine Empire, which he shipped to Yaroslav in Kievan Rus' for safekeeping. In , he left the Byzantine Empire, returning to Kievan Rus' to prepare to reclaim the Norwegian throne. In his absence the Norwegian throne had been restored from the Danes to Olaf's illegitimate son Magnus the Good. Magnus, unwilling to fight his uncle, agreed to share the kingship with Harald, since Harald in turn would share his wealth with him. The co-rule ended abruptly the next year as Magnus died: Harald became the sole ruler of Norway.
Domestically, Harald crushed opposition, and outlined the unification of Norway. Harald's reign was one of relative peace and stability, and he instituted a coin economy and foreign trade. Seeking to restore Cnut's " North Sea Empire ", Harald claimed the Danish throne, and spent nearly every year until raiding the Danish coast and fighting his former ally, Sweyn.
Although the campaigns were successful, he was never able to conquer Denmark. Not long after Harald had renounced his claim to Denmark, the former Earl of Northumbria , Tostig Godwinson , brother of English king Harold Godwinson , pledged his allegiance to Harald, inviting him to claim the English throne. Harald assented, invading northern England with 10, troops and longships in September , defeating the English regional forces of Northumbria and Mercia in the Battle of Fulford near York on 20 September.
