Cyber-physical systems relate to mission-critical infrastructure systems affecting the physical environment, such as power, water, wastewater, safety controls. These systems have traditionally relied on physical security and necessary firewalls as access control. Cyber-physical systems have long technology refresh cycles of 20 years or more, which undermines the ability to address vulnerabilities with engineering upgrades. Extended refresh cycles present a complex system engineering challenge. There is an operational need for cyber defense capabilities to defend cyber-physical systems from cyber-attacks. Systems engineering principles were applied in the concept development of the Department of Defense (DoD) More Situational Awareness for Industrial Control Systems (MOSAICS) Joint Capability Technology Demonstration (JCTD) to convert operational needs into an engineering-oriented view for the development of a prototype.