This book is the first to draw together the various strands of Irish national security policy and practice in a single chronological study, from independence in 1922 right up to the present day. Dr O'Halpin analyses the rapid emergence of a complex external security policy combining an absolute commitment to military neutrality and independent defence with close co-operation with Britain over issues of joint concern such as security and immigration. He traces the development of the army and police force in the new Irish state; and examines the state's reaction to the enduring republican threat, casting fresh light on how far the state was willing to put key constitutional protections into abeyance in its conflict with the republican movement. The book also examines the clandestine intelligence activities of belligerent powers during the Second World War, documenting the growth of the state's close wartime security understandings with the Allied powers, and the evolution of Cold War links with MI5 and the CIA. It investigates the evolution of post-war defence policy, and the activities of the defence forces in relation to the Northern Ireland crisis, as well as in their primary tasks of defending the state from external aggression and of contributing to UN peace-keeping operations. Dr O'Halpin highlights continuities as well as innovations in state security policy as the obligations and opportunities of European Union membership grate more and more against the absolutist rhetoric of neutrality. This book is essential reading for anyone wishing to understand the development of the Irish state in the twentieth century.