Given the challenges facing the cyberspace of the nation, this paper presents the tripartite theory of cyberspace, based on the status quo of cyberspace. Corresponding strategies and a research architecture are proposed for common public networks (C space), secure classified networks (S space), and key infrastructure networks (K space), based on their individual characteristics. The features and security requirements of these networks are then discussed. Taking C space as an example, we introduce the SMCRC (which stands for ‘‘situation awareness, monitoring and management, cooperative defense, response and recovery, and countermeasures and traceback') loop for constructing a cyberspace security ecosystem. Following a discussion on its characteristics and information exchange, our analysis focuses on the critical technologies of the SMCRC loop. To obtain more insight into national cyberspace security, special attention should be paid to global sensing and precise mapping, continuous detection and active management, cross-domain cooperation and systematic defense, autonomous response and rapid processing, and accurate traceback and countermeasure deterrence.