The United Kingdom has officially kicked off the process of ‘Brexit,’ almost nine months to the date after the country’s momentous vote to leave the European Union. British Prime Minister Theresa May formally notified the E.U. of her country’s desire to leave the bloc on Wednesday, setting in motion an unpredictable and fiercely complicated set of negotiations over the terms of departure and what comes after. In a letter to European Council President Donald Tusk, hand-delivered by Britain’s ambassador to the European Union Tim Barrow, May invoked “Article 50” of the Lisbon Treaty, the mechanism for departing the bloc.