Even Castlereagh, never a slave to public opinion, had noted that ‘if embarked in a War, which the Voice of the Country does not support, the Efforts of the strongest Administration which ever served the Crown would be unequal to the prosecution of the Conquest’. This was ‘our compass, and by this we must steer’. In other words, if such intervention was to occur, it was impossible for the British government to proceed without the consideration of humanitarian principles.

Non-intervention was appealing in abstract terms; it fitted the instincts of both Castlereagh and Canning. Subsequently, it has also proved attractive at times of war weariness and economic downturn: in the cash crisis of the 1820s and the wake of the Napoleonic Wars; in the 1930s, with memories of the Great War coinciding with the Great Depression; and certainly today, with the credit crunch occurring at the same time as ongoing and exhausting interventions in Afghanistan and Iraq.

Nevertheless, what dawned on the makers of foreign policy in the 1820s, as John Stuart Mill later put it was that, the ‘doctrine of non-intervention’, ‘to be a legitimate principle of morality’, as well as a successful strategy, ‘must be accepted by all governments ... despots must consent to be bound by it as well as the free States’. ‘Unless they do’, he concluded, ‘the profession of it by free countries comes but to this miserable issue, that the wrong side may help the wrong, but the right must not help the right. Intervention to enforce non-intervention is always rightful, always moral, if not always prudent’.