On July 1, 1967, the European Communities Merger Treaty came into effect, fusing the executives of the European Coal and Steel Community, the European Economic Community, and the European Atomic Energy Community (Euratom).
Although they were technically still known as the European Communities, with distinct rules governing each, some analysts consider this moment the real birth of the European Union. From then on, the Communities were governed by a single Commission (the supranational body representing the Communities as a whole), a single Council (the intergovernmental body representing the Member States), overseen by a common European Parliament, and answerable to a common European Court of Justice (ECJ) - four of the core institutions of today's EU.
A few months earlier, the United Kingdom had applied for the second time to join the European Communities, but as before, the French President, Charles de Gaulle, vetoed British accession. However, the UK's third membership bid was successful after de Gaulle resigned in 1969.
The UK negotiated entry from 1970-72, and its accession was legislated through the European Communities Act 1972 which incorporated Community law into the UK's domestic law and recognised ECJ jurisdiction. The UK formally became a member on January 1, 1973.
On March 29, 2017, the UK formally invoked Article 50 of the Treaty on European Union, beginning two years of negotiations before the UK leaves the EU.
Earlier this month, the May Government introduced the European Union (Withdrawal) Bill to the UK Parliament designed ''to Repeal the European Communities Act 1972 and make other provision in connection with the withdrawal of the United Kingdom from the EU''.
Although the Government likes to call it the ''Great Repeal Bill'', it is designed to make all the relevant EU legislation become UK law on ''exit day'', while removing ultimate ECJ jurisdiction.
So, in 50 years, we seem to have come full circle. But the European Union has changed hugely over that period. This partly explains the Brexit referendum result, but also the scale of the challenge of exiting the EU and developing subsequent relations with it. There can be no return to the ''independent UK'' that many Brexiteers lament.
Firstly, the European Union is far more deeply integrated than the EEC of half a century ago especially because of the new impetus that began in the mid-1980s: the formation of the Single Market, the Schengen agreement on open borders, monetary integration in the form of the euro, common citizenship, and symbolic relabelling as the European Union, committed to ''ever closer union''.
In fact, the UK opted out of key elements of these integrationist moves, but the rumblings of discontent from the Conservative backbenches and elsewhere about the abandonment of sovereignty to Brussels coincided with these developments and paved the way to Brexit.
The European Community that the UK joined in 1973 was also much smaller than the present-day European Union, consisting of a mere six members. Now there are 28 member states. Although UK governments were often strong proponents of enlargement, especially after the end of the Cold War, the increase in immigration to the UK that came with enlargement was a contributing factor to the rise of the UK Independence Party (UKIP) and ultimately the Brexit vote.
Yet it is these very factors that make it so difficult for the UK to extricate itself from the EU, and potentially so disastrous for it to try to do so. The EU is a massive market covering most of Europe. Its laws and standards are woven into everyday life in a way unimaginable 50 years ago.
And the original reasons for joining still hold. For example, even some keen advocates of Brexit are concerned that - recalling the Merger Treaty - exiting the EU now means coming out of Euratom, the institution that regulates the transport of nuclear materials among other functions and from which the UK nuclear industry benefits. And the fact the Republic of Ireland also joined the EU, at the same time as the UK, helped in the long-term to provide a basis for the Northern Ireland Peace Process.
With the republic remaining a member, and the UK leaving, the border between them may become even more significant than before they joined, with all the dangers that entails.
Nevertheless, some ''Europeanists'' welcome Brexit because it will remove an obstacle to deeper integration for the EU. But do people really want an ''ever closer union'', and what will be the consequences if it is foisted on them? The road to the EU of 2017 from that of 1967 is a complex one, but there is no doubt it has been an elite-led project going far beyond what much of the population wanted.
The humiliation of Greece raises fears that ''ever closer union'' is shorthand for conformity with neo-liberalism and punishment of those who transgress. And then there is the question of where the ultimate borders of the EU might lie.
Will it be an ''ever larger Union'' - give or take the odd Brexit or Grexit - and if so, how will the disagreements over Turkey or Ukraine be resolved?
