Hm, still not much of a response. Well, here goes the next step.
The Ninth Century, Part 6: The Bloodaxe War
1. The axe is sharpened
After Erik Bloodaxe died in 964, his son Harald Greycloak claimed sovereignty the entire kingdom - both the Irish lands that he himself ruled, and the islands to the west plus Eastanglia, ruled by his half-brother, Erik the Mariner. But Erik refused to submit to Harald as he had to their father. In response to this act of insolence, Harald prepared for war.
But Erik the Mariner was too sure of himself. His western lands - Ongelsey, Man, and the Hebrides - were vulnerable to attack from Dyflin and difficult to defend from his base in Östangeln. Harald attacked the islands and quickly secured the submission of their sub-kings. Erik turned to his son-in-law Thorkell of Jorvik (York) for aid, while Harald's mother Gunnhild appealed to allies in Orkney and in distant Denmark. All the islands prepared for a wide-spanning war.
2. The axe swings
Gunnhild traveled to Orkney, where Thorfinn Skullsplitter ruled as Jarl with his brother Arnkel. She promised to restore their former rule over some of the Hebrides in exchange for joining the war against Erik. A fleet from Orkney sailed against Östangeln in 965.
But Erik was not called the Mariner for nothing. Sailing out to meet Thorfinn, he sank the greater part of the Orkneyish fleet and forced the Skullsplitter to retreat. In days Erik outfitted a retaliatory expedition and sailed north to Orkney, hoping to obtain the quick surrender of his half-brother's ally. Erik failed to force a landing in the isles and fell back to the shore of Katanes (Caithness) on the mainland, where he and his men wintered.
In the spring of 966, Arnkel attacked Erik's positions in Katanes and was defeated. Erik had won, but his position was not strong enough to demand Orkney's outright submission to him. He settled for a promise of neutrality in the war and the establishment of his rule in those parts of the northern Hebrides that Harald had returned to Orkney.
Satisfied, Erik returned to Östangeln. He met with his son-in-law Thorkell of Jorvik, and they discussed the possibility of invading Ireland and dividing up Harald's conquests. Thorkell owed his position to Erik and saw in this new war the chance to make a name for himself, especially after his failed attempt to conquer Galicia, Spain, from his cousin in 964. He invaded the Isle of Man in 967 while Erik sailed - for the second time - against Cornwall, ruled by Sub-king Donyarth in vassalage to Harald. The Cornish were used to the ritual of surrender and homage to whatever Viking warlord happened to be sailing through, so by 968 Erik was using Kernow as a base to attack his brother's lands in Ireland.
3. The axe falls
The Skullsplitter had been no help at all, so Harald and Gunnhild looked elsewhere for an ally. They appealed to Dub, King of Scots. The Scottish Kingdom of Alba was probably the most powerful state in the British Isles. Celtic and Christian, Alba had been able to weather the Viking attacks that had brought down the rest of the archipelago, and since 900 the Scots and Vikings had been content to largely ignore one another. But in 967, Dub had an interest in helping Harald defeat Erik. Harald promised Dub all the islands off his coast should they win - Man, the Hebrides, even the Orkneys. Dub began constructing a fleet to attack the Hebrides and Man.
The Christian fleet took two years to complete. During that time, both Erik and Thorkell tried repeatedly to seize Dyflin and end the war. They failed every time. Erik was equally unsuccessful diplomatically: he was unable to find any powerful Irish chieftans, Celt or Norse, willing to turn against Harald. When it became clear that the Scots were preparing for war, Thorkell became a much less willing ally. He left his base on Man and returned to tend to his northern borderlands, gearing up for a Scottish invasion.
The Gaelic storm hit in the middle of 969. Dub and his fleet sailed to the Hebrides and then to Man, burning the Viking ships moored there. Although inexperienced in seafaring, their overwhelming numbers were too much for Erik's small but seasoned forces. Dub sailed home in triumph with plans to invade Jórvik itself in the coming months.
However, Scotland did not rest during Dub's absense. His succession to the throne had always been disputed by a number of relatives. Dub returned to find his cousin Cuilen securely on the throne with all the nobles supporting him. Cuilen had his cousin placed under arrest and later beheaded.
Cuilen thought Dub a fool for provoking the Jórvikish kingdom, a move he felt would lead to endless war with few lasting gains. He met with Thorkell in a field near their border and agreed to switch sides. Far better that the three kingdoms of Great Britain (Alba, Jórvik, and Östangeln) should band together and conquer the upstart in Ireland, Harald. Early in 970, three armies, led by Erik, Thorkell, and Cuilen, landed in Ireland. Harald Greycloak appeared to be out of luck.
4. The axe is bloodied.
There was one option left for the heir of Bloodaxe. Gunnhild was descended (or claimed to be) from the long-dead king of Denmark, Gorm the Old. She would now appeal to her distant cousin, Denmark's current King, Harald Bluetooth. She begged him to come to her aid, offering all sorts of honors and lands should he agree. He did. Gunnhild sailed into Dyflin at the head of a vast fleet of ships lent by the king.
After driving the invaders off of Ireland, Harald Greycloak decided to go after his brother's base in Östangeln. With Danish help, he was able to secure a number of towns and fortresses there. In 972, Harald Bluetooth himself arrived from Denmark with additional troops. His gamble seemed to be paying off, and he stood to seize a fine prize in England.
By 973, the Haralds had conquered Östangeln and driven Erik off the island. Harald Greycloak swore the requisite oaths of loyalty to Harald Bluetooth of Denmark. Harald B now had a foothold in England. He pressed on to the important trading city of Lundun, for decades considered "shared" between Jorvik and Östangeln. His forces took it after a long seige. To stem the tide, Thorkell met with the Haralds in December. In return for peace, he paid a hefty ransom to the victors. Harald Bluetooth received not only Lundun, but Essex and Kent as well. The growth of the Danish Empire had begun.
Cuilen, a cautious ruler, did not like the prospect of facing Dyflin and Denmark alone. He returned home, bringing with him all his forces. In the end, the Scots gained only Man and the southernmost Hebrides, which had been conquered by his late cousin Dub back in 969.
5. The axe is shouldered
The Haralds persued Erik to his refuge in the Orkneys, which he fled in 974. A compulsive wanderer, he went to the Faeroes and later Iceland, where he died c. 980.
Harald Greycloak was secure in his Irish kingdom that he had helped conquer and enlarge in his youth. He also bore the title Sub-King of Östangeln, although there he was not free to act without permission from Erik Bluetooth in Denmark.
Although the war had begun as a succession crisis between two pagan Vikings in England and Ireland, the real winner turned out to be Christian(izing) Denmark. Harald Bluetooth gained two new territories in the British archipelago, Orkney and Östangeln, both of which contained more territory than they had a decade earlier. Harald set the stage for great conquests in England by his successors, Sweyn and Cnut.
The Celtic states of Britain came through the war shaken but intact. Scotland had in fact gained territory, while Cornwall managed, once again, to slip through the cracks of power and hang on to its independence, if only for a little while.
The other winners of the war were the skalds and their audiences, who for centuries thereafter would be entertained and amazed by the sagas written about it. The Saga of Erik the Mariner is considered the finest example of Ongellseyan poetry, and many other fine epics emerged from Ireland, England, Man, and Orkney. Erik's reputation traveled with him to Iceland and later to Greenland, where even more fantastical accounts of the long war were composed and later written down.