Yesterday was a significant day for Europe. While Britain and the EU formally agreed a Brexit transition deal, a hostage crisis unfolded in France. Terrorism is one of the challenges facing the continent; the transition deal shows what Europeans can accomplish when they work together. Because Brexit does not signal a rejection of Britain’s European history or geography, as if that were possible, but a desire to reset relations – and once we have left the EU, we seek a constructive partnership. Its necessity has been clarified by alarming developments in conflict and trade.

Three weeks ago, in Salisbury, a nerve agent was used in Europe for the first time since the Second World War. The EU stands...