At the end of the Irish War of Independence, Dublin signed an unsatisfactory treaty with London that, amongst other things, required oaths of allegiance to the British Empire. To many this was a price worth paying, but for others it was impossible. Very quickly, in 1922 the country collapsed into a cruel civil war that split organisations like Sinn Fein and the IRA, local communities, and families. It was less devastating than some other European civil wars but it left a ghastly number of dead, injured and immiserated across Ireland, north and south. And it cast a long shadow across Ireland. Fine Gael and Fianna Fáil, the two parties that grew out of the rival factions, have ruled Ireland since the end of the civil war. It was only in 2019 – almost a century after the conflict – because of Sinn Fein’s electoral success that the two parties could see their way to officially working together. Drawing on completely new sources, Ireland’s most brilliant historian shows how important this tragic war was for understanding Ireland now.