For the first time in the history of the European Communities and the European Union, a member has served notice that it is quitting. Predictions of a European Union crack up have flown around for years, but this is the first sign of it happening.

As a result of the referendum held in the United Kingdom, votes in the Parliament to implement the expressed wishes of the people, and finally the assent of the Queen, the UK Government has done it. Prime Minister Theresa May sent a letter to European Council President Donald Tusk.

May’s letter officially notified Tusk and the European Union of the UK’s intent to withdraw from the European Union, invoking Article 50 of the Treaty of Lisbon to force the rest of the EU to negotiate a settlement.

May specifically included EURATOM, the European Atomic Energy Community, in this letter, even though EURATOM was never fully integrated into the rest of the EU. Therefore the UK is withdrawing from EURATOM’s nuclear fusion research and loan guarantee programs, and will allow EURATOM later to be merged into the EU.

As of now, the United Kingdom is still in the European Union, but there is now legal force to require the EU to negotiate a settlement, one which will only require a “qualified majority” to put into effect. It is not possible for one obstinate EU member state to block the process.

The only question left is: Who’s next?