Our constitutional system was carefully designed to prevent any one branch from seizing too much control over the entire government. Only Congress can write legislation; only the President can execute the laws; only the courts can judge whether the laws are constitutional.
This balance of powers, however, does not maintain itself. It is a dynamic equilibrium requiring each branch of government to protect and fully exercise its rightful authorities. When one branch encroaches on another, that balance is endangered — and so are the freedoms the separation of powers were intended to protect.