This United States Army Manual, Training Circular TC 3-12.2.90 Mission Thread Defense February 2024, provides a methodology for units to apply systems thinking to mission assurance, focusing specifically on cyberspace and cyber dependencies. This publication serves as an aid for units to understand system functions as a part of mission analysis to support planning and the Army design methodology. The target audience for this publication are the staffs of units requesting, directing, planning, or conducting defensive cyberspace operations. It will also aid all those responsible for cybersecurity or assuring missions with significant cyberspace components or dependencies.