On June 23, 2016, a majority of voters (51.9 percent) in the United Kingdom's four nations–England, Northern Ireland, Scotland, and Wales–voted in a nationwide referendum to leave the European Union (EU), a move that would sever a relationship begun in 1973, when the body was the European Economic Community. May was not prime minister at the time of the referendum but took leadership of the governing Conservative Party not long after David Cameron's consequent resignation and has committed the country and the government to go forward with the political, economic, and social divorce.

The U.K. government on March 29, 2017 officially invoked Article 50 of the EU Constitution, which stipulated that the country must give up its membership in the EU within two years. That timeline officially ends on March 29, 2019.

That is now a hard and fast date on which all ties will be severed, though. Both the U.K. Parliament and the EU Parliament must approve any agreement between the entities, and the terms of any future relationship between the two will certainly provide the basis for negotiations taking place long after the "official" date.

The EU has made public its desire to have the details wrapped up by the end of October 2018, so that all EU members have enough time to study the arrangement and then the European Parliament can vote on the proposal by the March 29, 2019 date. A total of 72 percent of member states must approve the arrangement, as must the European Parliament and the U.K. Parliament.

The U.K. and the EU began formal negotiations on June 26, 2017, and some serious questions remain, including these:

• What to do about trade agreements with the other 27 EU member states. The "open borders" of the EU allow free trade across the membership; presumably, the U.K. would have to negotiate new trade deals with every EU member with which it still wanted to do business
• How to restructure financial arrangements that have the U.K. and other EU member states intertwined to a great degree
• What do about the border between Ireland and Northern Ireland; the entire island became part of the U.K. in 1801, and a partition divided the island in 1921. Some have discussed reviving border checks
• How to pay for the new border enforcement apparatus that will be needed in the U.K. after Brexit takes hold. The current government budget allocates a significant amount of money, but that's only for a year.