a fundamental truth or proposition that serves as the foundation for a system of belief or behavior or for a chain of reasoning. "the basic principles of Christianity"
synonyms: truth, proposition, concept, idea, theory, assumption, fundamental, essential, ground rule
a fundamental source or basis of something.
"the first principle of all things was water"
What she's saying there is that policy has to be based on what they believe in or it's not going to work.
What she meant by it was that invading a Sovereign state required a scaled response. You can't let that type of aggression go unchallenged, or others might see it as a weakness and act similarly.