《捍卫王国》的出版在出版业界是史无前例的：为纪念英国军情五处成立一百周年，该机构首次授权独立历史学家接触其机密档案。本书揭示了军情五处在20世纪英国史中发挥的确切作用，从1909年10月英国陆军上尉凯尔建立该机构，随后经过两次世界大战，到如今还发挥着反间谍反恐的作用。本书讲述军情五处是怎样运作的，它与政府间维系着怎样的，在哪些方面成功了又在哪些方面失败了。作者对这一切发表了畅快淋漓的评判。

本书也披露了先前不为人知的英国的敌人的身份，他们的行踪被该机构揭穿了，极大增加了我们对很多著名事件以及臭名昭著的人物的了解，无疑破解了很多长期的谜团。最重要地是，揭露了英国先前最隐秘组织的所在地。鲜有书籍能让我们对上个世纪的英国史即刻了解那么多。

To mark the centenary of its foundation, the British Security Service, MI5, has opened its archives to an independent historian, the first time any of the world's leading intelligence or security services has taken such a step. "The Defence of the Realm", the book which results, is an unprecedented publication. It reveals the precise role of the Service in twentieth-century British history, from its foundation by Captain Kell of the British Army in October 1909 to root out 'the spies of the Kaiser' up to its present role in countering Islamic terrorism. It describes the distinctive ethos of MI5, how the organization has been managed, its relationship with the government, where it has triumphed and where it has failed. In all of this, no restriction has been placed on the judgements made by the author. The book also casts new light on many events and periods in British history, showing for example that through well-placed sources MI5 was probably the pre-war department with the best understanding of Hitler's objectives, and had a remarkable willingness to speak truth to power; how it was so astonishingly successful in turning German agents during the Second World War; and that it had much greater roles than has hitherto been realized during the end of the Empire and in responding to the recurrent fears of successive governments (both Conservative and Labour) and or Cold War Communist subversion. It has new information about the Profumo affair and its aftermath, about the 'Magnificent Five' and about a range of formerly unconfirmed Soviet contacts. It reveals that though MI5 had a file on Harold Wilson it did not plot against him, and it describes what really happened during the failed IRA attack in Gibraltar in March 1988. When Rab Butler was appointed Home Secretary with responsibility for the Security Service in 1957 he didn't even know where its headquarters were. "The Defence of the Realm" now describes this previously extremely secretive organization more fully than any previous book - and identifies all its main buildings on the end papers.