What it says is this: In order to avoid giving one department (such as the Department of State or the Department of Homeland Security) too much power — because too much power in one place is a bad thing — you should allow each department to defend itself lawfully against any other department that it thinks might be figuratively stepping on its toes, or invading its turf.

It was a very good idea, but we don’t really have such a system today. Because state and federal agencies (“departments”) are under the executive branch, they are not allowed to fight with each other over principles. They usually have to do what the President or Governor tells them to do, or the person in charge gets repaced.