Three caveats before we begin:
This list constricts itself to those episodes of history in which the British themselves participated. The victories of Charles Martel at Tours and what was left of the Roman Empire at Châlons were arguably of greater importance than most of the below – in keeping Europe a Catholic continent, they prevented Britain from becoming a total outcast in the world’s affairs and ensured that the thread of common religious identity connecting the Roman Conquest and the Norman Conquest was, while frayed, ultimately unbroken.
Secondly, the below is a martial list. I do not subscribe to the historical school which believes that the experience of the 11th century ploughman menaced by the armies of Stephen and Matilda is as deep and rich in historical lessons as those of the great Kings and Queens. The only important lesson to draw from such lives is that for the vast part of history, life for all but a tiny elite was laborious, miserable and painful, and that it is not now even for those who fondly pretend that it is. History is made by ideas, the ideas of men tested in blood. This list acknowledges such.
Finally, of necessity this list is concerned with the island of Britain, rather than with the United Kingdom – Irish, particularly Northern Irish, history would require an essay of its own, and quite a painful one for an Englishman to write, at that.
Here we go, then. The events of history which have shaped modern Britain:
1. 43 – The Roman Conquest of England and Wales
The current political and cultural landscape of Britain has remained relatively steady for almost two thousand years. The borders of the Roman empire have become the fissures of the modern day British isles. While ethnic and cultural communalities between the Scottish, Welsh, and Cornish persist to this day, only the Scottish have consistently maintained a distinct political identity over a roughly continuous geographical area since Roman times.
Conquest affected England and Wales differently. England was settled whereas Wales, aside from Carmarthen, was fortified. England enjoyed, for the first time, the benefits and refinements of continental trade and political association, while the tribes of Wales noticed the occupation largely in terms of the extent of the dislocation from traditional customs that it brought about, given the massacre of the druids. When the Romans left, the English had become reconciled to being a continental people, the Welsh to being occupied but not subdued, and the Scottish to the solitary aim of their national policy ever since, which has been to resist subjugation from the south. So, to some degree, it has remained for each for most of the time since.