Ethelred the Pious
The Kingdom of Kernow, or Cornwall, emerged in the Sixth Century as a breakaway state or sub-kingdom of the Kingdom of Dumnonia. The Saxons called it "West Wales". By the late Ninth Century the kingdom was coming under the dominion of Wessex, and much of its land was owned by West Saxon priests. The gradual Saxon takeover of the little country was halted by the Vikings' conquest of England in the 870s.
The Norse conquest
By the 890s, neither Kernow nor England's southwest were yet under the control of the new ruler, the King of Jorvik. In 904 King Hogni invited Hrolfr the Northman, a Danish chieftain, to settle in the country of Dafna and subdue it for Jorvik. Dafna (Devon) was just across Kernow's eastern border.
Hrolfr made quick work in conquering his new jarldom, and soon was looking to expand his personal power. In 904 he led an army into Kernow. Rather than simply add Kernow to his jarldom, Hrolfr saw the value in allowing it to remain totally separate from Jorvik. He placed his 11-year-old son Viljhalmr, or William, on the Cornish throne.
In 924, Hrolfr backed a losing candidate for the throne of Jorvik. Thereafter he steadily lost influence in England and concentrated more on his Cornish kingdom. Vilhjalmer continued these policies after his father's death. In the 930s, he began his own journeys of expansion, adding the isles of Adreney, Wernsey, and Jarsey to his kingdom, then landing on mainland Europe to take the fortress of Carsborg.
In order to maintain his independence, William often had to play along with foreign powers and accept vassalage to them: this he did with Jorvik in the 940s and Erik the Mariner, ruling from Ostangeln, in the 950s. This made Kernow part of the scattered empire of Erik's father, Erik Bloodaxe, which meant that it was to be fought over when the Bloodaxe's sons fought over their inheritance.
The turbulent era of Doniert II
Vilhjalmr died in 960. His son was known only by his Cornish name, Doniert II. Like his father, he was a Christian and was quite integrated into Cornish culture - even more so since by the time he assumed the throne, his family's title in England was being challenged, and other nobles had seized control of much of Dafna. Doniert maintained the Cornish kingdom on both sides of the Channel. During his reign, the Channel Islands were settled with Cornish farmers. He had to fight several times to defend Carsborg and its peninsula against incursions from both Angelania and Neustria.
The Bloodaxe War of 964-974 also threatened the kingdom. It pitted Erik the Mariner, based in Ostangeln, against his brother Harald Greycloak, king of Dublin. Harald was in a much better position to rule Kernow, and as soon as he came to the throne he secured Doniert's fealty as a vassal. Erik sailed to Kernow in 967 to conquer it for the second time. Doniert offered no resistance, hoping to stay out of the war. He capitulated to Erik immediately and offered ships and men. In 968 crews of Kernowmen joined Erik in raids against Dublin. After that the fighting shifted away from the western sea as Scotland and Denmark were lured into the war. Doniert successfully slipped through with his kingdom intact and independent.
Around 980 Doniert made an alliance with Guy I of Neustria. The two kings agreed to help each other: Cornish ships would help with Guy's war in Brittany, while Neustrian money would help pay for defense against Angelanian raiders. Doniert transferred his feudal allegiance from Ostangeln to Neustria, and Guy agreed to let him keep Carsborg.
Vassal of England
This period of Kernow's separation from England ended around 1000, when for the first time England became a single kingdom under Sweyn Forkbeard. Doniert's son-in-law Osvalt was now king, and he had little choice but to acknowledge the overlordship of the Danish conqueror. King Cnut kept the same policy when he came to power in 1018, but he like his father was too concerned with affairs in Scandinavia to concern himself with Kernow. Kernow's status continued as it had since Saxon times: it was a semi-independent kingdom under the suzerainty of a powerful neighbor.
By this time, the nobles of southern England were finally taking an interest in Cornish land and were beginning to encroach on its border. Madron, Kernow's king during the time of Cnut, began to look for outside allies. Good relations continued with Neustria, and the little Cornish exclave was maintained in vassalage to that Frankish kingdom. Madron also sought connections with petty kings in Wales and Ireland, which like him were under English influence hoping to maintain their independence.
The Welsh Rebellions
England's disputed succession upon Cnut's death presented an opportunity. During the chaos of the Cnutsson's War, the rival kings were not able to pay attention to Kernow or their other Welsh vassals, and the local kings began to dream of independence. The first round of the rebellion was fought in 1043-1047 in the distant kingdoms of northern Wales and was put down by England's new king, Olaf I of the House of Kent.
In 1051, the kings of Wales joined together in a larger rebellion. This time, Kernow joined them. King Conan, called the Strong, defeated an invading English army in 1052, bringing a great surge of confidence to the rebels. Cornish forces helped drive back a second army sent into southern Wales the following year. The rebels finally laid down their arms in 1058 after Olaf crushed the northern kingdoms of Gwynedd and Ongellsey. But they accepted a very loose vassalage. Conan and the other southern kings did not lose their thrones, and in subsequent years could boast that they were unconquered.
Under the Kentish kings
The next sixty to seventy years are remembered as a peaceful time in Britain's history. Olaf and his two sons, Sidroc and Woldemar, ruled free of major wars or upheaval.
This was not necessarily good for the kings of Kernow, or in Wales more generally. For centuries their independence had depended largely on England's being distracted by its own turmoil as various dynasties from throughout the Nordic world vied for power. Now that the kingdom was firmly united under the benevolent rule of a line of native English kings, the Welsh would have to confront ongoing efforts to curtail their freedom and control their affairs.
By this time, Kernow's own ruling dynasty was purely Welsh in its language and outlook. The kings certainly descended in part from Hrolfr and his Viking band, but they did not consider their house to be his. Nevertheless, Kernow maintained a strong military and naval tradition for a kingdom its size, and this could be traced right back to Hrolfr and his Vikings. Hrolfr's conquests across the Channel were not yet lost, either. The islands remained Cornish throughout the medieval period, and Cornish fiefs around Carbourg remained for centuries, though in vassalage to the kings of Neustria.