I read a quote from over two decades ago from a younger person, seemed like about college age, about what a shame it was in medieval times that people didn't realise what a waste of money war was. She stated confidently that if some group had just refused to go to war and put all that money into making things for the people like roads and bridges they would have prospered so greatly that they would outcompete all their neighbors quickly, "and maybe the others would catch on and everyone would do that." We can all see that the young woman could just as easily have come from 1965 or 1985 or our own era. There was a variation of it when discussing going to war in Iraq and Afghanistan, that if we just built roads and schools for them instead they would be more likely to be friendly. In that case, the fact that we had built schools and roads and were building more was quite clearly not general knowledge.
There is a huge amount taken for granted about how optional wars are. America is one of the powerful ones, so our wars have some element of optionality about them, and in a few places in history there have been other entities that had the same freedom. On the other side of that equation are the clear aggressors in history who could have decided not to invade their neighbors. Unfortunately, that is not always easy to see. The jostling, claims, and counterclaims were continuous, and those who did not invade were likely to be invaded.
If you were Aquitaine or Wales or Lombardy there was no choice that said "we're going to put everything into trade and inventing stuff because wars are so horrifically expensive." Nor is there an escape by claiming they could have just consented to be ruled by their neighbors, because even if the taxes were very heavy they would still come out ahead by not paying for war. Those neighbors didn't come in and take over your government, leaving you to go on as before. They sold you as slaves, because slaves were one of the most valuable commodities, and every nation had people who could be sold. There were those in Britain who thought becoming part of the Roman Empire in the time of Julius Caesar might not be a bad idea, and the Romans were big on planting the idea in many places that there were lots of advantages (so just surrender, please. Sign here.) When it became clear that the cost of that was going to be taking lots of slaves and people being switched from growing food for themselves and local trade to extractive technologies and export goods, 'most everyone changed their minds.
This is not news to anyone here, but I take the time to write it because even we treat the idea with only mild impatience, something that looks reasonable at first but when one looks closer reveals itself to be unworkable. We also take so much for granted in our peaceful ages with wealth far above subsistence and freedom to move where we like and work at what we will. To treat it lightly, as a mere bad idea, is almost as crazy. In those times, and in virtually every time and place where population density created competition for resources, the idea would have been immediately recognised as stark raving mad. Only in the 20th C did people have enough peace and prosperity that they could talk themselves into such foolishness.