Theresa May’s political weakness was brutally exposed to Brussels on Monday as an agreement struck between Britain and the EU to solve the problem of the Irish border and move to the next phase of Brexit talks was torpedoed by a last-minute telephone call with the leader of the Democratic Unionist party. Confidence early on Monday that an agreement was within reach came to nothing when, during a working lunch with the European commission president, Jean Claude Juncker, May was forced to pause discussions to take a call from Arlene Foster. The unionist leader, whose party currently provides the Tories...