Why then is there a constant battle over the meaning of defence?
Because it does matter. In this world of war and turmoil people must still seem legitimate. And in this world legitimate and lawful action has begun to share the meaning of the word defence. Why? Well that is obvious. There are almost no mandatory aggression treaties left. All mandatory obligations that exist are defensive. So fighting on the defence makes us more likely to be defended.
This has grown and grown and today we see the total extreme of this when the New Sith Order engaging in conflict on the basis of a legitimate and widely recognised Causus Belli feels the need to frame the war as a defensive one. This is despite there being no mandatory defence clause in any treaty held by Kaskus and a large number of very powerful alliance that would bail out the New Sith Order if they were to face any aggression from another.
I find this strange. Often the wars that are regarded as most legitimate are the ones most regarded as legitimate actions. The declaration on NpO a year ago was regarded by many as a legitimate action. It was not a defensive one. IRON's declaration of war on LSF was not a defensive war. However it was a legitimate action. NSO declaring war on Kaskus was legitimate. However for some reason they now feel the need to frame it as a defensive war.
We can also see examples of very aggressive wars framed as defensive wars. This is especially so when they are not legitimate. The Dave War began with MK claiming to be acting in defence. GOONS often claims to be fighting in defence under its claim to be fighting rogue unaligned nations rather than small alliances.
What is the result? We have changed the meaning of the word legitimate so that people now believe that as soon as a war is defensive it is legitimate and we seem to have allowed the concept of a just war become secondary to this. In some ways this is good. It prevents mindless aggression. It keeps war relatively at bay. However it also serves as a paralysis. An alliance with an honest and good cause will struggle to overcome the barrier that the treaty structure creates, whereas one with a weak cause but a nominal military offense against them (however twisted) may feel itself safe.
This tangled web we weave has made legitimacy a second place concept. And now we must live with that.