Why do nations break into one another's most important computer networks? There is an obvious answer: to steal valuable information or to attack. But this isn't the full story. This book draws on often-overlooked documents leaked by Edward Snowden, real-world case studies of cyber operations, and policymaker perspectives to show that intruding into other countries' networks has enormous defensive value as well. Two nations, neither of which seeks to harm the other but neither of which trusts the other, will often find it prudent to launch intrusions. This general problem, in which a nation's means of securing itself threatens the security of others and risks escalating tension, is a bedrock concept in international relations and is called the 'security dilemma'.