War, in the age of the ‘Revolution in Military Affairs (RMA)’ has changed its nature and tactics. Use of sophisticated systems has made it difficult to penetrate defences. Cyber war, however, appears as a drill to penetrate the defences of nations, because defence against cyber offence is so far not strong. Clarke was the first cyber security advisor to US president George W. Bush (Jr.) from 2001 to 2003 and Knake was a senior officer in the Department of Homeland Security (DHS). This book primarily focuses on US cyberspace security based on their experiences.