John Grainger examines the long and erratic process by which the British Isles was gradually (and as it turns out, temporarily) unified over the course of eighteen centuries, and the subsequent beginnings of the process of disintegration, manifested in an independent Ireland and increasing devolution to, and nationalism in, Scotland and Wales. Taking the Roman (partial) conquest and forming of the province of Britannia as his starting point, he outlines the major stages by which unification was brought about, through invasions (or in reaction to the threat thereof) and the vagaries of dynastic succession. James I was the first monarch to reign simultaneously over the whole British Isles but full political union was not completed until the Act of Union that came into effect on 1 January 1801, against the backdrop of war with France. It was maintained for just 122 years before the Republic of Ireland gained independence in 1922. John Grainger sees the granting of their own parliaments to Wales and Scotland as further stages in the process of disintegration, which may be accelerated by 'Brexit'.