Cybersecurity in national defense protects critical systems and data from threats. It involves strategies like encryption, firewalls, and intrusion detection to safeguard military networks and operations from malware, phishing, and advanced persistent threats. The field has evolved from early cryptography to addressing complex challenges like IoT vulnerabilities and nation-state attacks. Key concepts include confidentiality, integrity, and availability, while future trends point to AI-driven defenses and international cooperation in cyberspace.