I am no psychologist (I promised myself to read more on the very interesting cognitive psychology), but I have a suspicion that warmongers have a desire for war, military bullying and general boasting of national strength that is not related to common cost/benefit thinking.
They just like the idea of national military strength and of using this strength. It appears to give them satisfaction in itself. This would also explain why and how they can consider even disastrous conflicts "successful": They gave them satisfaction. Everything else appear to be a mere pretence.
I don't think countries should spend billions or go to war just to satisfy this population share's desire for it. Maybe a modified Flottenverein model could be a solution: Whoever wants more military spending than required for national and collective defence should join the 'club of offensive military sponsors' and pay club fees.
But unlike the Deutscher Flottenverein, these fees would not be used to lobby the government for more military spending: The club would instead pay for the additional, unnecessary spending which serves only the warmongers' taste.
Now if smart and economic theory-educated warmongers would come to this blog and comment, they could put a lot of argument pressure on me with my cost/benefit approach: Warmongers' desires for offensive military capability and actions is in econ theory a "preference".
Preferences are a kind of dark matter to economists; it would take god-like mind reading skills to learn about them directly. That's why economists need to retreat in many, many cases (more than they actually do) to a position of "We don't have enough information for a calculation, so the best approximation to the ideal is to let preferences show themselves in a vote".
A warmonger could attack my writings like this (but no smart or educated enough ones come to face me, so I play devil's advocate here). My next line of defence against their warmongering would then be to point at the votes/polls which oppose war and huge military budgets and an examination of the votes/polls which do support it.
And that's where we're at political science and psychology research, for there are plenty ways how a vote can be manipulated to not reveal the best interest of the voters (or the population as a whole).
This is the real area of activity of warmongers; they don't calculate cost/benefit, for it never seems to support their case anyway. Instead, they try to manipulate populations into supporting their agenda, so they can satisfy their antisocial desire.
And this is where societies need to meet, counter and resist them as long as possible. This resistance fades in strength as memory of failures dwindles and inexperienced people replace experienced people, so warmongers will eventually succeed in some countries, but we could at the very least lengthen the intervals at which they succeed.
It's an ever-lasting conflict with a particular kind of dangerous idiots.