When Edward 'Longshanks' became king of England in 1272, why did he take the throne as Edward I instead of Edward II, given that England had at one point been ruled by Edward the Confessor (1042-1066)? Was the Norman conquest considered to have marked a decisive break in state continuity?

The short version is that Edward I was not known by that number upon taking the throne. He was just known as King Edward. He was named after Edward the Confessor - his father Henry III was a big fan of the saint - and was also the first post-conquest monarch to bear a traditional English name. However, since Edward the Confessor had lived over two centuries before there was no fear of confusing the two Edwards and a reference to King Edward would by default mean the current one. It is also worth noting that Edward “Longshanks” would properly be Edward IV, after Edward the Elder, Edward the Martyr, and Edward the Confessor. The title Edward the First would come much later.

Even upon Edward’s death his title as the first was up for debate. An early thirteenth century text referred to Edward the Confessor as “St. Edward, King of the English, third of his name”, and after his death, Edward “Longshanks” was referred to as Edward the Fourth by at least three authors – the earliest writing in 1307 about the king’s death and referring to “the passing of the great king, Edward the Fourth”. However, this trend was replaced by later events.

The thing is, for the most part you didn’t need to use regnal numbers unless you had multiple monarchs with the same name in rapid succession, and the post-conquest kings had most avoided this problem until the arrival of the Edwards. Edward “Longshanks” was succeeded by his son Edward (side note, Edward was the youngest of his father’s sons, and England almost had a King Rufus instead), who was usually referred to as King Edward, King Edward the Younger, or King Edward the son of King Edward. It was only when his son and heir was also Edward that things got complicated. Saying King Edward son of King Edward son of King Edward is a mouthful, so the convention developed to refer to Edward III as King Edward, the third after the conquest.

For reasons that aren’t entirely clear this practice was adopted into the parliamentary record which essentially made it official. Edward III’s grandson and heir Richard II was known as King Richard, the second after the conquest and his usurper was known as King Henry, the fourth after the conquest even though no English king pre-conquest was named Richard or Henry. Eventually the ‘after the conquest’ was dropped, although it seems to have been fairly common until the time of the Tudors.

It is also worth mentioning that there is an alternative naming convention for these three monarchs that excludes numbers entirely. Before they were crowned they were more often known by their place of birth, e.g. Edward of Westminster/London, Edward of Caernarfon, and Edward of Windsor. During his lifetime Edward the Black Prince was instead known as Edward of Woodstock, in keeping with this tradition.

As to whether the Norman Conquest was considered a definitive break, that’s a whole topic we could spend hours on, but in terms of just the numbering it is actually kind of a yes and no situation. Robert Bartlett has argued that by naming his son Edward, Henry III was deliberately choosing to highlight the continuity of the English throne from pre-conquest. This was one unbroken line of monarchs whose names repeated. In contrast, however, the late medieval English by choosing to use a post-conquest numbering system sided with the idea that the conquest was a dividing line between the old and the new monarchy.

Most of this answer is based on Robert Bartlett’s work in Blood Royal: Dynastic Politics in Medieval Europe which has an absolutely fascinating chapter on regnal numbering – highly recommend if that’s the kind of thing that interests you. The English are by no means the messiest or weirdest case of medieval numbering!