The phrase "plan of defense" is a common term used in military and security contexts. There are several synonyms that can be used to describe the same concept, including "strategy of protection," "tactic of safeguarding," and "scheme of fortification." Other synonyms for "plan of defense" include "approach of security", "program of protection" and "scheme of defense". Each of these synonyms conveys the idea of a planned approach to protecting against potential threats and risks. Ultimately, selecting the right synonym will depend on the specific context in which the phrase is being used and the nuances of the situation at hand.