The British government yesterday notified the European Union of its intention to leave. This will be a big moment in a tragedy; it will be a tragedy for Britain, but it will also be a tragedy for Europe. It is an appalling way to celebrate the EU's 60th anniversary.
Even if the exit negotiations go well, the decision to leave will have huge consequences for Britain. Economically, it will lose favourable access to by far its biggest market. Politically, it will create great stresses inside Britain. Strategically, it will eject Britain from its role in EU councils. Britain will be poorer, more divided and less influential.
Brexiters will deny all this. They are wrong. The evidence on modern trade is clear: Distance is of enormous importance. The supply chains that link physical goods and services together work best over short distances. The models on which Brexiters rely ignore this reality. This is also why the creation of the single market required substantial regulatory harmonisation, which allows relatively frictionless cross-border trade. Brexiters will discover, too, that all trade deals impose constraints on national autonomy and the more market-opening the deal, the tighter the constraints.
Brexiters will also learn that geography is political destiny. Britain can never be a non-European country. It will always be intimately affected by developments on the continent. But now, faced with a threatening Russia, an indifferent US, a chaotic Middle East, a rising China and the global threats of climate change, it is removing its voice from the system that organises its continent. Britain is no longer in the 19th century. It is in the 21st. Isolation will not be splendid - it will be isolation.
The departure of Britain is also a tragedy for Europe. Britain has long been the standard-bearer for liberal economics and democratic politics. It is one of the continent's two strongest military powers. It has close links to the English-speaking countries. It has a global perspective. It has, at least until now, been pragmatic. Its views on what would benefit the EU (the single market and enlargement) and what would harm it (the single currency) were right.
Only someone ignorant of history would dream that Europe would be more prosperous, stable, influential, democratic and liberal if the European Union shattered into 28 national pieces. The system of nation states has repeatedly proved unstable. In this case, with the United States increasingly withdrawn, the EU's collapse might lead to a struggle for hegemony between Germany and Russia or, worse, a pact between them at the expense of weaker neighbours. If the EU does survive, as I hope, Germany will dominate. The Germans do not want this. Why do the British?
Yet, Brexit is going to happen, thanks to Mr David Cameron's folly in agreeing to the referendum, mismanaging the negotiations and bungling the terms of the referendum itself. Going through with Brexit is not a constitutional necessity; the referendum is not binding. But it is a political one: The Conservative Party would shatter without it.
But the mood of the negotiations and their outcome are still to be determined. We know they are going to be complex and difficult. We know that the process of withdrawal and deciding upon the details of a new relationship are not going to be completed inside two years. But we do not know how these negotiations will be approached. This is not so true on the EU side, where priorities are clear, as on the British side.
Reaching a deal is a necessity. This is most obviously true for economic reasons: seeking to obtain better market access from relatively unimportant markets, while suffering a large deterioration in the terms of access to Britain's most important markets, would be ludicrous. Failure to reach a deal on money owed, treatment of persons, shared institutions, the nature of future trade arrangements and the transition to them would poison future relations. Britain would be the bigger loser: The impact on Scotland might be terminal for Britain. Yet, the effect of a brutal divorce on the EU would also be large.
If a deal is to be reached, Britain, as the weaker party will need to make concessions, starting with the money owed. That is not only the sensible thing to do. It is the right thing to do. The country has obligations that come from more than four decades of membership. As a civilised and trustworthy country, it must fulfil them.
This means, in turn, that Prime Minister Theresa May must be prepared to make a stand against those who desire no deal at all. The EU's negotiating position is a reasonable one. Britain must be willing to reciprocate. It must make concessions to ensure a harmonious and cooperative relationship in future.
Mrs May has stated that "I am clear that no deal for Britain is better than a bad deal for Britain".
Let us hope that she does not believe this. Failing to reach a deal would be bad for everybody. Mrs May has no mandate for the threat she has made of turning Britain into a low-tax, minimum-regulation country. The internal divisions such a strategy would bring would make those created by the referendum look like a mock battle compared with a real one. Britain certainly needs a deal, but so does the EU. The tragedy would be far worse without one.
I no longer hope that Brexit can be avoided. That does not mean it needs to be welcomed. Still less does it mean that it does not matter how it happens. The Prime Minister must reach a deal that preserves as much as possible of Britain's economic, political and strategic relations with the EU. History will judge her by how much of this she achieves.