Someone who is an expert in warfare should probably be able to provide good answers to questions about military-defense. If party A exercises warfare against party B, then it stands to reason that party B will need to exercise military-defense unless party B is prepared to immediately and unconditionally surrender to party A (which, come to think of it, is also a defense strategy, though unlikely to work well in the long term). Hence, the two are merely opposite sides of the same coin, but one can reasonably expect people to look for one or the other depending on the focus of the particular question they are writing.
Sounds to me like a good candidate for a tag synonym.
I certainly do not see any need for an additional, different tag for military-defense which is distinct from warfare, as we currently have. The two can probably be merged (and, in fact, will need to be, if we want to establish the synonym).