Bangladesh, Sept. 21 -- For decades, the United Kingdom has been held together not only by law and institutions, but by an assumption: that despite political disagreements between its constituent nations, the union itself remained the stable framework within which those disagreements would be resolved.

That assumption is now being tested.

On September 14, political leaders from Scotland, Wales and Northern Ireland gathered in Cardiff and issued an unusually coordinated declaration asserting their nations' right to determine their constitutional futures. Scotland's John Swinney, Wales's Rhun ap Iorwerth and Northern Ireland's Michelle O'Neill, alongside Sinn Fein leader Mary Lou McDonald, used the occasion to demand that Westminster sho...