Watching the slow-motion train crash that is the Greek debt crisis invites the question as to whatever happened to European solidarity. Listening to politicians in Berlin explain that parsimonious German voters will not stomach a bail-out of their spendthrift continental cousins offers only half an answer.

There is more to the story than an angry collision between Greek profligacy and German moral superiority. Behind the proximate threat lies a more unsettling truth. The crisis is symptom as well as cause. For all its upheavals, there used to be something reassuringly ineluctable about the European Union. Now the enterprise is beginning to unravel.

Greece's predicament, and the response of its eurozone partners, holds dangers on many levels: a sovereign default within the single currency; contagion as markets test the resilience of Portugal, Spain and Ireland; and a breakdown of the political trust and mutual support mechanisms on which the monetary union depends.