The taking of measures necessary in times of emergency and/or for the defence of the realm including the control of armed forces.

In Chandler v Director of Public Prosecutions [1964] AC 777, Lord Reid stated: 'the disposition and armament of the armed forces are and for centuries have been within the exclusive discretion of the Crown …' (at 791).

In Burmah Oil Company Ltd v Lord Advocate [1965] AC 75, installations owned by the oil company in Burma (then a British colony) had been destroyed during World War Two on the orders of the commander of the British forces, to prevent them falling into the hands of the Japanese. By a 3-2 majority the House of Lords held that compensation was payable to the company, there being no general rule that the prerogative could be exercised without compensation. Lord Reid stated: '[T]he prerogative certainly covers doing all those things in an emergency which are necessary for the conduct of war.'