Bangladesh, Sept. 20 -- 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. Scotlands John Swinney, Waless Rhun ap Iorwerth and Northern Irelands Michelle ONeill, alongside Sinn Féin leader Mary Lou McDonald, used the occasion to demand that Westminster shou...