A new view of the Norman Conquest of England by UEA historian Prof Tom Licence based on papal letters shows that the concept of a 'legitimate war' goes back a long way.
Analysis shows that King Harold may have forfeited his crown long before the Battle of Hastings by backing the wrong side in a bitter struggle between two rival popes.
And that the outcome of 1066 may have been shaped as much by political conflict in Rome as by events on the battlefield at Hastings.
Prof Tom Licence, from UEA's School of History and Art History, said: "1066 was not simply decided on the battlefield at Hastings. The outcome may also have been shaped by decisions taken almost a thousand miles away in Rome."
Papal approval of a war was the medieval equivalent of a UN resolution - not that they matter much these days - but you get the point.
Prof Licence argues that this interpretation helps answer a longstanding historical question - why would a pope support the violent overthrow of an anointed king?
Probably for the same reason as the West supported the overthrow of a democratically elected government in Ukraine - because it suited their political purposes.
The Pope did not really care about the spiritual welfare of the English just like the West did not really care about democracy in Ukraine - the parallels are obvious.
The provocation of the Ukraine war was legitimised as a means to lead the country towards democracy just as William's conquest was legitimised as a way to turn the English away from 'satan'.
But it's still interesting to know that these kind of geo-political machinations were alive and well in the 11th century just as they are today.