Divide and conquer is a well-know strategy in software design and architecture. In terms of OOA/OOD it is not really my favorite approach, but this is not the topic of this post.
Divide and protect is one option to secure large distributed systems. The concept of Divide and Protect is about the compartmentation of a system into functional blocks with identical requirements in terms of security and privacy. It supports a defense in depth strategy, and it helps to handle the complexity of large installations. The compartmentation of a given system leads to security zones with different levels of trust that should be outlined in a digram. Based on such diagram (red = not trusted, .., green = trusted), the system architecture can be developed in a comprehensive manner. This is especially true for the communication architecture and the selection of appropriate protocols. By using this approach, non-functional requirements can be addressed in the early beginning of the product development process which means no change (requests) later on.