In the 2016 referendum campaign, one of the few moments of comedy was the sight of English champions of Leave struggling to get to grips with the complexities of the Irish border. Less comic have been the consequences: the creation of an economic frontier down the Irish Sea and the spectacle of Protestant gangs rioting across Northern Ireland, ostensibly over the diminution of their British identity. Admittedly, the land border between north and south is incomprehensible: it follows few natural boundaries, bisects farms and villages and even runs through houses. When it was established in 1921, cities such as Londonderry were cut off from their economic hinterlands in Donegal and decades of communal strife followed. Yet the border has survived and will ‘celebrate’ its centenary this month.

In The Partition, Charles Townshend explores the origins of the formal division of this long-divided island. As he points out, before there was a border, Protestants and Catholics clashed over the territory through which it would later run. During the 19th century, local traditions of communal violence in the north