The UK joined the EU in 1973…
One of the reasons the EU was created back in 1952 (it was then called the European Coal and Steel Community), was to prevent another major European war from happening. The economic successes ‘spilled over’ into some political fields as well, and the countries integrated more and more. Other countries looked at this and wanted in so they could also reap the benefit of a larger market with no internal boundaries. Nowadays, the EU is much more than economic cooperation, but the internal market is still seen as the cornerstone of European integration.