Boris Johnson took office as British prime minister on Wednesday after an audience with Queen Elizabeth, a moment that put an avowed Brexiteer in control of Britain’s exit from the European Union for the first time since the shock 2016 referendum. Johnson enters Downing Street at one of the most perilous junctures in post-World War Two British history – the United Kingdom is divided over its divorce from the EU and weakened by the three-year political crisis that has gripped it since that referendum. One of Britain’s most prominent Brexit campaigners, Johnson has repeatedly pledged to leave the European Union by Oct. 31 – “do or die” – and to inject a new optimism and energy into the divorce, which he argues will bring a host of opportunities. But his Brexit strategy sets the United Kingdom up for a showdown with the European Union and thrusts it towards a potential constitutional crisis, or an election, at home. “We are going to get Brexit done on Oct. 31 and we are going to take advantage of all the opportunities it will bring in a new spirit of can do,” Boris Johnson, 55, said on Tuesday after he was elected by Conservative Party members. To implement Brexit, Johnson will appoint Dominic Cummings, the campaign director of the official Brexit Vote Leave campaign, as a senior adviser in Downing Street. Wednesday’s events combined arcane British political choreography with the realpolitik of appointing a new government – likely to be heavy on Brexit supporters….