The United States is committed to an open, secure, interoperable, and reliable Internet that enables prosperity, public safety, and the free flow of commerce and ideas. The Internet was not originally designed with security in mind, but as an open system to allow scientists and researchers to send data to one another quickly. Without strong investments in cybersecurity and cyber defenses, data systems remain open and susceptible to rudimentary and dangerous forms of exploitation and attack. Malicious actors use cyberspace to steal data and intellectual property for their own economic or political goals. Governments, companies, and organisations must carefully prioritise the systems and data that they need to protect, assess risks and hazards, and make prudent investments in cybersecurity and cyber defense capabilities to achieve their security goals and objectives. Behind these defense investments, organisations of every kind must build business continuity plans and be ready to operate in a degraded cyber environment where access to networks and data is uncertain. To mitigate risks in cyberspace requires a comprehensive strategy to counter and if necessary withstand disruptive and destructive attacks. The United States Department of Defense (DoD) is responsible for defending the U.S. homeland and U.S. interests from attack, including attacks that may occur in cyberspace. This book examines the DoD's cyber security strategies; provides US Cyber Command with strategic direction to ensure unity of effort as duties are performed in the service of the nation; and discusses international strategies for cyberspace.