The Normans under William the Conqueror were the first usurping party to import foreign mercenaries and foreign concepts of rule to the island which had been unified for several centuries under the very effective and solid Anglo Saxon system of rule. After 1066, peace was never really known again on the British Isles. The Norman ruling family constantly fought among themselves and allied with various continental powers whenever it suited them.
The first great Anglo Saxon diaspora took place in 1075, when Edgar Etheling's followers took a great number of Saxons and Danes who refused to submit to Norman rule to the Balkans, where they became fighters for the Byzantine Empire in their own war against the relentless Norman incursions onto their domain. The Saxon warriors were so fearsome and skilled that they were incorporated into the Byzantine Varangian Guard in no time and were given a large tract of land in the upper Balkans to settle. The Saxons called this land Nova Anglia, or New England.
The second and most important Anglo Saxon migration away from the Isles took place between 1603 and 1780 in the form of the founding of the 13 Colonies. A far greater number of Anglo Saxons along with Welsh, Scot, and members of religious groups persecuted by the Crown moved across the Atlantic and were instrumental in founding the United States of America. Many bluegrass and folk singers will tell you that the Appalachian mountain country is the identical sister to the highlands of Scotland and Ireland, identical in both physical features and spiritual connection. The Appalachian country can be considered an offering from God to the rugged Scots and Irish whose original homeland has been perverted and defiled by a Crown that has strayed from the teachings of the Good Book. The rugged over-mountain men of the Appalachian country who dealt the death blow to Cornwallis's forces in 1782 were the direct descendants of the Anglo Saxon warriors who refused to submit to Norman rule and allied with the Kingdom of Alba (Scotland) to continue a guerrilla war against the continental invaders until 1075. In essence, the American Revolution had avenged the tragedy of the Battle of Hastings and the oppression that occurred afterward. The War of 1812 and the slaughter of the final British invasion force at New Orleans sealed the message for the invader. The message is: "You do not belong here. You had one chance, and you got an island, and you defiled and perverted it. Now you have reached too far and you will not be given another chance again. Leave now while you still can, because only more death awaits you if you try to head inland again."