The American Civil War: brother against brother. The English War of the Roses: mutually destroying the Kingdom. The War in Iraq: Muslim against Muslim. The Scots, the Welch, the Irish - they were all so busy fighting, clan against clan, they could not hold out against the Danes, the Normans and later, the English.
It was Norman against Norman at the battle of Val-es-Dunes in 1047. In the uprising of Counts from the east, the rebels were confident they would put William's cousin, Gui of Burgundy, on William's throne. None of the rebels had expected King Henry of France to come with ten thousand soldiers to support William. With the entry of the King into their dispute, they no longer outnumbered William's force. Worse yet, they became traitors to the crown if they continued the fight.
Ralph of Tesson had sworn, with others, upon the saints of Bayeaux, to "strike at William where ever he found him". What a quandary! To keep his oath, he must break his oaths to his King. While the two sides hovered opposite each other, Ralph spurred his destrier across the field, shouting his war cry. He rode up to Duke William and hit him with - his glove!
He returned to his men to keep faith with his King. They attacked the rebels from the rear as King Henry and Duke William attacked with their combined force to the front. In the battle that followed, the rebels suddenly lost heart, and fled towards the river Orne. Thousands were killed in battle, countless others drowned trying to cross the river.
My 28th great-grandfather, William Duke of Normandy won. My 27th great-grandfather, Ranulf, Count of Bayeaux, was killed in battle.
We won. We lost.
Ref: The History of the Norman Conquest of England: Volume 2, by Edward Augustus Freeman, Carendon Press 1870; digitized Nov 3, 2007.