From at least the mid seventeenth century, and arguably at least from the Renaissance onwards, one principle has been widely seen as the dominant way of securing ‘order’ in the chaotic and anarchic world of interstate politics, in Europe to begin with and later the world. This principle became known as the ‘balance of power’. As Hume famously remarked, however, ‘it is a question whether the idea of the balance of power be owing entirely to modern policy or whether the phrase only has been invented in these later ages’.1 Many have seen the operation of something like what we would today call the balance of power working in all ages, or at least in all ages and places where something resembling a’states system’ has existed.2 Whether that is true or not, however, it was certainly during the eighteenth, nineteenth and twentieth centuries that the principle reached its apogee as a response to the ‘problem of order’ in the context of the European states system and in the twentieth century in particular it became the centrepiece of the most protean and widely discussed approach to international relations of all, to wit, political realism.