The preface for Simpson's book states ‘During the Second World War a very considerable number of people were detained by the British government without charge, or trial, or term set, on the broad ground that this was necessary for national security. Most were not British citizens, but technically enemy aliens – in fact most of these enemy aliens were refugees from Europe. A far smaller number of those detained were British citizens, and they were held under Regulation 18B [of September 1939, and more particularly the specially formulated section 18B (1A) introduced in November] of the Defence Regulations; it is with this regulation and those detained under it that this book is concerned’. The background to the implementation of the regulation was the threat of an imminent German invasion of Britain in May 1940 following the collapse of Allied forces in France. Under the legislation the Home Secretary of the day - initially Sir John Anderson, subsequently Herbert Morrison - was free to detain and imprison as he saw fit anyone against whom evidence of potential disloyalty, untested by legal process, was presented by the security services. The normal safeguards against abuse of executive power traditionally available to British citizens, such as the provisions of Magna Carta, Habeas Corpus and trial by jury, were effectively suspended. The regulation remained in use long after it became clear that no organised Fifth Column existed in Britain.

Most of the British citizens detained were members of Fascist or extreme Right-wing groups, who were generally opposed to the war with Germany. In May 1940 there existed a fear of a potential ‘Fifth Column’ movement which might undermine the British war effort, such as had destabilised the Republican defence of Madrid during the recent Spanish Civil War, during which the phrase was coined, and more recently during the Nazi invasion of Holland. But the implementation of a measure associated with totalitarian and not with democratic government inevitably raised serious civil rights issues, and was justified only by the extreme danger of the time. Those affected were arrested not for offences against the law but for what they might conceivably do. Although a Home Office Advisory Committee was set up to oversee internment, individuals once arrested had little chance of redress, and could be kept in prison indefinitely with no attempt to charge or try them with any offence. The evidence on which they were arrested was secret and sometimes, as Professor Simpson shows, of dubious accuracy. Some individual detainees undertook legal action in the courts against the Home Secretary under Habeas Corpus or for wrongful imprisonment, but such actions almost invariably failed. The words which form the main title of Professor Simpson’s book are those of Winston Churchill who, originally a strong supporter of the regulation, came later to recognise its danger to democratic freedom and who described it as ‘in the highest degree odious’.

As the most prominent Fascist group active in Britain at the time, Sir Oswald Mosley’s movement British Union (whose full title was ‘ British Union of Fascists and National Socialists'), which had campaigned vigorously against the war up to and beyond its outbreak, was particularly affected by the measure, with many leading members arrested and interned. Both Mosley and his wife, Lady Mosley (Diana Mosley) were imprisoned. Mosley was released only in November 1943 when he developed serious health complications. Various other similar groups and individuals were similarly affected. Strong pressure was applied subsequently by political opponents to maintain internment even after the danger of invasion had long receded, whilst some in government sought to prolong its use even after the end of the war, though in the event it was abolished the day after VE day.

Professor Simpson's book discusses the history of the Regulation and its implementation, its significance for civil liberties, the more prominent legal cases to which it gave rise and the parts played by the Home Office and MI5. It is based on extant official records, so far as these are now available, and on interviews with many of those directly or indirectly affected or involved, and others. Research has been hampered by the fact that many of the relevant government records have been destroyed, whilst access to other records was even at the time of writing still not allowed. In-depth attention is given to the cases of Benjamin Greene and R.W. Liversidge. Correspondence with Lady Mosley is also on file. In addition there is an appendix on another case of the time, though not directly relevant to the internment of British citizens, that of the spy Tyler Kent, an American citizen who worked at the U.S. Embassy in London. Kent stole highly sensitive documents relating to communications between Churchill and Roosevelt.