In March, another former MI6 chief, Sir Richard Dearlove, wrote that “the truth about Brexit from a national security perspective is that the cost to Britain would be low”. Perhaps it was this that provoked the Sawers response. But the reality is that any limited costs and benefits of Brexit in domestic security terms must be seen as secondary to the profound threat to peace in Europe posed by the EU’s fixation on a common foreign and defence policy for all its members, including the United Kingdom.

By trying to create its own foreign policy and its own military forces – which on typical European levels of defence investment will remain modest indefinitely – the EU risks reverting to the uncertainties of the pre-Nato era. After all, why is Nato the most successful military alliance in history? The answer is clear: it is the deterrent effect of US membership. This means that, unlike in 1914 and 1939, a would-be aggressor must face the prospect of war with the world’s most powerful state right from the outset since Article V of the Nato Charter requires that any attack on a Nato member must be seen as an attack on them all.