The Norwegian Invasion of Denmark was a brief conflict between the Kingdoms of Norway and Denmark that saw the complete defeat of the Danish army by the forces of Norway. As a result, Norway gained control over Denmark and eventually began further expansion into Sweden and Ireland.
Prior to his expedition to claim the throne of England, Duke William of Normandy made an agreement with Holy Roman Emperor Henry IV and King Sweyn II of Denmark for support. Despite only being able to give meagre troops and supplies, William agreed to defend Denmark from Norway in the case that the latter ever attacked the former. However, after the end of the War of the English Succession , the Peace of Westminster required that William of England not take up arms against the Kingdom of Norway for twenty-five years. Seeking both revenge for the losses his men faced fighting the English and Normans, as well as the pursuit of uniting the Scandinavian realms under one king, Harald Hardrada raised a new army in 1071, crossed the Skagerrak Strait, and invaded Denmark.
War on Land
In June 1071, having raised an army of 8,000, King Harald Hardrada crossed the Skagerrak and landed at the beaches of Fladstrand. Hardrada refused to bring much supplies with him to Denmark to make the trip faster and so fewer ships could carry more men. Instead he relied on a burnt-earth strategy of pillaging local villages for supplies, the most notorious of such cases being Hjorring, where the entire village was not only ransacked, but burnt to the ground for resisting the Norwegians. After hearing of the horrors that befell Hjorring, the town of Hirtshals surrendered to the Norwegians with very little resistance. From there the Norwegians march to, and conquered, the towns of Thisted and Norresundby with relative ease.
Sweyn II, very shaken by the rapidity of the Norwegian assault, raised an army of 3,000 men and marched it north to prevent a successful Norwegian crossing of the Limfjord. At narrow sections of the fjord, the Danish had torn apart some of their longboats and several bridges in a desperate attempt to halt the Norwegian advance. To avoid fighting the brunt of Sweyn's army just yet, Harald marched his army from the Norresundby area to Thyborons, where they built a large pontoon bridge across the fjord and continued to march south. This actions forced Sweyn to march an already tired army back down the Jutland peninsula to Viborg.
From Thyborn, the Norwegians marched to the large town of Holstebro, where Hardrada had planned to meet some longboats that could take his men down the Stora River. However, after several defeats at sea by the Danish fleet, his navy was unable to meet him, forcing Hardrada to continue their march by land. Deciding to meet the enemy and crush them on land, where he had a greater advantage, Harald and his men marched to Viborg, where they encountered Sweyn's army en force. A large battle ensued, but by the end of the day the Norwegians had crushed the Danish army by a combination of numbers and the skill of Hardrada's archers. Defeated, Sweyn marched his forces back to Vejle.
Following the defeat of the Danes at Viborg, the Norwegians again began their campaign of pillaging and stripped the Danish countryside of necessary supplies. After the fall of Horsen in February 1072, Sweyn again feared that the Norwegians would defeat him in battle, and thus retreated to the island of Funen. Having seen their king betray them readily to save himself, and hoping to escape the wrath of Harald Hardrada, many of the towns left on the Jutland peninsula willingly surrendered to Norway. Having successfully employed Thorgils Ericson's strategy of naval attrition, Hardrada could easily cross the Baltic Sea and fight Sweyn on land again.
After retreating of Odense, Sweyn travelled to Nyborg to avoid another Norwegian attack. However, Sweyn's plans to cross the sea again to Korsor failed as the ships he had prepared for that venture were again defeated by Thorgils Ericson. Having run out of options, Sweyn called on all the men of Funen to fight for their king, raising a force of 2,500 to fight Hardrada. When the Norwegians reached Nyborg they were at least surprised to meet a larger force than expected. However the experience of Hardrada's troops made the difference and as Sweyn had put the militiamen at the front of his force to keep his troops rested, he set himself up for defeat. The Norwegians attacked the militiamen, who began to fall back after taking moderate casualties, many began to flee to avoid being trampeled by others. The resulting confusion lead many of them to retreat into the lines of Sweyn's real troops, further adding to the confusion among the Danes. With a single, frontal assault on the Danish troops, Hardrada managed to win the battle, and Sweyn surrendered himself to the Norwegians. In return for sanctuary, Sweyn agreed to surrender the rest of his realm to the Norwegians.
War at Sea
After the Danish victory at the Battle of Svoldor in 999, the Kingdom of Norway had been inferior in naval warfare to the Kingdom of Denmark. Hoping to recover from that defeat, King Harald Hardrada appointed Thorgils Ericson to lead in the rebuilding of the Norwegian war-fleet. Despite some advances, Thorgils still had to rely on the use of longships for his navy, but had built a fleet of eighty large longships for use in fighting the Danish fleet, as well as many smaller, faster ships. Their first engagement was at a battle off the coast of Laeso, where a Danish fleet intercepted a Norwegian force planning to raid down the River Gudena. While the Norwegian fleet had more men and larger boats, the Danish were more experience at sea-fighting after raids in the Mediterranean Sea. The resulting battle saw eight Danish longships defeat two Norwegian "warboats" and ten longships by outmanuevering them.
After his defeat off of Laeso, Ericson and his fleet limped back to Norway, gathering a larger force and sailing back to Laeso. There they met a Danish fleet of near-equal strength, and were once again forced to retreat after mounting losses, this time returning to Fredeikshaven, having recently been captured by Hardrada. After receiving re-inforcements from Norway again, Thorgils sailed his fleet arond the peninsula to the entrance of the Stora River, hoping to assist in the capture of the area. However, another Danish fleet met him towards the beginning of the river, blocking it and leaving Ericson unable to travel farther down the river. Furious at their repreated failures in 1071, Thorgils travelled back to Norway for the winter, preparing for the return of his fleet in the spring.
Once the icy waters of the Baltic Sea began to thaw in March, Thorgils sailed his large fleet once more into Danish waters, meeting them off the island of Anholt. This time Thorgils employed a strategy of attrition, his fleet had grown in numbers over the winter and now he was able to secure a victory over the Danes by sheer numbers. Over the remainder of the spring of 1072, this strategy not only won the Norwegians more naval battles, but also slowly eroded the numbers in the Danish fleet. In May 1072, off the town of Kalundborg, Thorgils Ericson won a sweeping victory against the remnants of Denmark's fleet, cutting off King Sweyn II from retreat to the east. After his surrender following the Battle of Nyborg, Sweyn II also surrendered the remaining parts of his fleet to the Norwegians.
Following the surrender of King Sweyn II of Denmark in June 1072, King Harald Hardrada made an agreement with Henry IV for Sweyn's sanctuary within the Holy Roman Empire. Fearing possible transgression, Henry IV also agreed to pay Hardrada a sum of gold in order to keep the Norwegians out of the Holy Roman Empire. Hardrada accepted the gift despite not having any plans for an invasion of German territory, but soon after died in Oslo in 1074 from natural causes. Meanwhile, Thorgils Ericson, also late in life, personally funded a series of ports along the Norwegian coast to allow for the building of a larger fleet for the Kingdom of Norway. Thorgils also brought in Italian and Byzantine shipbuilders to learn how to make larger ships called galleys for the Norwegian Navy. The growing power of Norway's navy and the apparent mastery of the Baltic Sea would soon have repercussions in much of Europe.