The United Kingdom was the world's foremost power during the 19th and early 20th centuries, most notably during the so-called "Pax Britannica"a period of totally unrivaled supremacy and unprecedented international peace during the mid-to-late 1800s. The country continued to be widely considered a superpower until the Suez crisis of 1956, and this embarrassing incident coupled with the loss of the empire left the UK's dominant role in global affairs to be gradually diminished. Nevertheless, the United Kingdom remains a great power and a permanent member of the United Nations Security Council, a founding member of the G7, G8, G20, NATO, OECD, WTO, Council of Europe, OSCE, and the Commonwealth of Nations, the latter being a legacy of the British Empire. The UK had been a member state of the European Union (and a member of its predecessors) since 1973. However, due to the outcome of a 2016 membership referendum, proceedings to withdraw from the EU began in 2017 and concluded when the UK formally left the EU on 31 January 2020, and the transition period on 31 December 2020 with an EU trade agreement. Since the vote and the conclusion of trade talks with the EU, policymakers have begun pursuing new trade agreements with other global partners.