Prime Minister Theresa May set the UK on course for leaving the European Union in two years’ time, a divorce that will redefine the country’s relationship with its largest trading partner and end decades of deepening political integration on the continent. “After nine months the UK has delivered,” EU President Donald Tusk, said in a Twitter post at 1:28 pm in Brussels after receiving a hand-signed letter from May, which invoked Article 50 of the Lisbon Treaty, the formal exit mechanism. May told lawmakers in London shortly afterwards that “this is a historic moment from which there can be no turning back. I choose to believe in Britain and that our best days lie ahead.” Her tone was more conciliatory than a speech earlier this year laying out her priorities, as she stressed the desire to forge “a deep and special partnership” with the EU and said the UK will do all it can to help the bloc of 27 “prosper and succeed.” But an immediate clash was evident over the sequence of negotiating, with the prime minister calling for discussion of the future partnership alongside the terms of exit and Tusk saying that talks will focus on “all key arrangements for an orderly withdrawal.” Nine months since Britons unexpectedly voted to leave, the phony war of what Brexit might look like will turn into a real battle over the complex terms of a settlement with 27 other governments. Enough lines over money, trade and immigration have been drawn to suggest a tough process with costs to both sides. “We’re moving from a UK monologue to a hard negotiation,” said Gregor Irwin, chief economist at Global Counsel, a consultancy, and a former official in the UK Foreign Office. “There are going to be many upsets along the way.” The pound erased losses made earlier on Wednesday against the dollar. It was little changed at $1.2457, having earlier touched a one-week low of $1.2377.