Forty-four years ago, the United Kingdom joined the European Union. Nine months ago, in a stunning outcome, a slim majority of U.K. voters opted to leave in a national referendum. On Wednesday, Brexit formally began when Britain's envoy to the EU hand-delivered a letter from Prime Minister Theresa May to European Council President Donald Tusk informing him of London's intention to leave the union and commencing a two-year negotiating process over the terms of Britain's withdrawal and their future relationship. Both Europe and the U.K. have entered uncharted waters, and the talks and the outcomes are likely to be bruising for both.

Britain seeks to regain sovereignty over core national concerns, in particular to reclaim control over immigration and exempting itself from the jurisdiction of the European Court of Justice, while retaining access to the European market. As May has argued, "While we are leaving the institutions of the European Union, we are not leaving Europe." European governments, while hoping to maintain good relations with Britain, do not want to signal a readiness to accept "a la carte" membership — permitting members to pick and choose their obligations and benefits. Too favorable a deal could encourage other governments to follow Britain. The EU position, as explained by Tusk, is simple: "To minimize the costs for the EU citizens, businesses and member states." That means that at a minimum, Britain cannot get a better deal in the future than that which it enjoys today.

While negotiations over the terms of Britain's withdrawal and subsequent relationship will be extremely difficult, that challenge is magnified by London's desire to work out details of both simultaneously. EU and European officials counter that only after withdrawal has been agreed can they then consider their future relationship. In addition, elections that will be held this year in France and Germany will slow the negotiating process as those two governments will be central to shaping the European position in the talks. Since the EU treaty stipulates that the process must be finished within two years — a period that includes ratification of the deal by EU member states — time constraints will have a powerful impact on the outcome.