The existential challenge to the Union began with the election of a disciplined cadre of MPs from the Irish Party in 1880, and culminated with the election of a Scottish National Party majority government in 2011. The years from 1885 to 1921 have been described as the high age of primordial unionism. For a primordial unionist, the Union of Great Britain and Ireland, passed by an Act of Parliament in 1800 and taking effect in January 1801, was to be defended at all costs. Prime Minister William Gladstone's conversion to Home Rule in 1885 splintered the party systems throughout the British Isles, and in Scotland, created the Unionist Party, the dominant party until the 1950s. This article examines the challenges faced by the Union, focusing on efforts to kill the Home Rule during 1885–1924 and the passage of the Scotland Act 1998. It also discusses the Constitutional Convention, the Labour Party government and its policies, and the ‘West Lothian Question’ and ‘Barnett Formula’.