If rights are "whatever men agree they are at a given time and place" then your rights are at the mercy of the majority of people tomorrow (a given time) in your city or neighborhood (a given place), instead of being inalienable.
Perhaps I am reading him wrong but Griffin says that rights are "secured by military power" not given by military power. Whether Griffin agrees or not, since I can't tell for certain from just this paragraph you quote, it was not merely in the Bill of Rights that the principles were written, after they had "defeated the armies of Great Britain". It was long before military victory was assured that the Declaration of Independence was written, setting forth the reasons for the colonies to revolt and setting out the philosophical convictions that emboldened the colonists to oppose tyranny to the risk of life and limb.
It would be wrong to conclude that military power has anything to do with creating rights, only securing them. It is an important distinction, otherwise preemptive force would be justified for the creation of any new "rights", and there are many new rights that people would love to start a fight to obtain.
THAT is a very dangerous idea.