A possible explanation, in the form of a letter to The Spectator (which they did not print):
Sir: Douglas Murray cannot seem to fathom why David Cameron supports Turkey’s membership of the EU (‘Cameron’s support for Turkey’s EU membership should worry us all’, 15 March).
The Prime Minister has long been an advocate of free trade across the EU, but I don’t put him down as a committed Eurofederalist. My guess is that Mr Cameron favours Turkish entry into the EU for precisely the same reason that many Tories favoured EU enlargement during the 1990s, namely that it would serve as a bulwark against further political integration.
Mr Murray frets over the possibility that 75 million Turks could soon enjoy visa-free access to the Schengen Area. Perhaps Mr Cameron is betting that mass immigration of Turks would prove so disagreeable that Turkish accession, were it to be achieved, would undermine not only Schengen but also the principle of free movement itself.
Noah Carl,I'd be interested to know if anyone else has an alternative explanation.
Nuffield College, Oxford