Saying "la defensa de ese país está fuerte", sounds a little off. Perhaps this comparison might help. La defensa de ese país es fuerte--->The defense of that country is strong, referring to its quality, we know for a fact that its defense is strong. La defensa de ese país está fuerte---->This sounds really akward in spanish. This would imply that its defense is strong right now, as in, being strong.
In American English it is spelled defense. In British English it is defence.
As an aside: I recently learned a new word in English "retronym". Defined as a newer name for an existing thing that differentiates the original form or version from a more recent one. It is thus a word or phrase created to differentiate between two types, whereas previously (before there were two types) no clarification was required. (Wikipedia) "British English" fits that definition.
I agree: "That country's defences are strong" is for me the most natural way to express this idea in English. Using the military paradigm, we talk about a country having defences, not defence. Defence is the verbal noun.
Ok, defence (sing.) is fine if we're talking about football. I would argue that military is a more general interpretation of this sentence than football.