The United Kingdom left the European Union's orbit on Thursday, turning its back on a tempestuous 48-year liaison with the European project for an uncertain post-Brexit future in its most significant geopolitical shift since the loss of empire.

Brexit, in essence, took place at the stroke of midnight in Brussels, or 2300 London time, when the United Kingdom ended the de facto membership, known as the transition period, which lasted 11 months after it formally left on Jan 31...