This book contains, in slightly extended form, the Wiles Lectures which I delivered in the Queen's University, Belfast, in May 1976. My first and pleasant duty, therefore, is to express my gratitude to Mrs Janet Boyd and the other Trustees of the Wiles Foundation, for inviting me to the lectureship. They thus did me a signal honour, and also supplied me with a motive for presenting in generally assimilable form some results of my research and reflection over the last ten years.

An important feature of the Wiles Lectures is that a number of scholars, eminent in fields connected with the topic chosen, are invited to attend and to lead the discussion which follows each lecture. I was very fortunate in those who were invited to hear and to comment on my lectures. Mr B. J. Bond corrected my account of Clausewitz in a number of important places, and also alerted me to the flow of important books on Clausewitz which were to appear in the succeeding six weeks. (I refer to those of Professor Raymond Aron and Professor Peter Paret, and to the new translation of On War by Professor Paret and Professor Howard.) Professor J. J. Lee had already called my attention to the extensive literature in German on the Marxists' reception of Clausewitz; for which I am greatly indebted to him. My gratitude to Professor A. J. M. Milne and Professor Ernest Gellner is of a deeper and more general kind. Both have unfailingly encouraged me in the researches which lie behind this book.