There are plenty ministries of defence, secretaries of defense et cetera. All of them used to be called differently, such as "Ministry of War", but then countries began to pretend for a while that waging war was for outlaws only.
Ever since, the talk about "defence policy" and similar has been deceptive, for much of it wasn't about defence at all - or about preserving the peace. Much of it was about foreign policy, bullying, enforcing UN embargoes, UN blue helmet missions, military observer missions and the like.
This is regrettable, for it enables the pro-intervention camp to deceive the public about what military spending is for; they pretend it's for "defence" (or "defense"), when in fact much of it isn't, thus isn't that necessary, and some funds would probably be withheld if the public wasn't deceived through language.
In essence, this is similar to the manipulative rebranding efforts such as rebranding "anti-abortion" into "pro-life". Public support differs on one and the same question depending on how you frame and call it.
I suppose true defence - the prevention of aggressions against the own camp (country or alliance) through deterrence (si vis pacem para bellum) - is not only legitimate, but also undisputably justified if the odds of success are acceptable. To some countries this may be more about foreign policy than about military spending (see Iceland, Costa Rica).
You will not be able to determine the exact budget required to keep the peace or to be able to repel an invasion, stop a bombing campaign or break a naval blockade, of course. Yet if there's an all-knowing entity and it would tell us the figure, we should all be able to agree that this figure is about true defence spending.
Additional spending on the other hand is not about true defence - it's about offensive capabilities. It's not about breaking a blockade, stopping a bombing campaign or repelling an invasion - it's about the capability to do this to other countries. Well, this and some more obvious luxuries, such as preserving some ancient warship or having some hospital ship or two.
Again, we cannot tell where exactly this luxury spending on aggressive capabilities begins, but I insist it's nevertheless worthwhile to remember the distinction between true defence and offence.
The practical utility of this distinction is given because sometimes it's all-too obvious that some expenditure is not related to defence at all. A mini aircraft carrier for Thailand, for example. A containerized military base camp system for Germany. Another supercarrier for the U.S. Navy, with U.S. allies having the majority of non-U.S. carrier forces. A submarine conversion for launching a hundred or so cruise missiles.
We can tell with a simple look at these that they are not for true defence but for additional military capabilities. Yet we can only do so if in our mind we understood the difference between true defence and offence. Without this distinction, all these non-defence expenses would still be registered as "defence", and I've seen uncounted articles making this exact mistake, resulting in pseudointellectual garbage.
In theory we could determine the optimum amount of non-defence military spending with a cost/benefit analysis. Again, it's not possible -particularly not in advance - to pinpoint the optimum budget exactly. Yet again the knowledge about the suitability of cost-benefit analysis may help us, for again there will be cases where this criterion certainly will not be met. We would be helpless in face of claims that a spending proposal is fine unless we have a suitable criterion (such as cost/benefit) in mind for at least an own estimate.
Let's pretend our country spends ten billion € per year on the capability to invade a defended beach with a division of marines, using hovercraft, helicopters and landing boats. Now we could look for what value is generated by this capability, we would look at history as empirical evidence and find not a single case where anything even only approaching such a capability was required since the demise of Imperial Japan. Every single example was substitutable by our army, including the landing at Inchon 1951, which the Japanese had already done without dedicated ships and troops in 1904. Our estimate should thus be that such a capability stands no chance to pass a cost/benefit comparison even though we don't know the exact figures.
Meanwhile, someone who buys into 'All of this is defense' is a mere Kool-aid consumer who lacks the capability to spot the difference between true defence and other military spending.