Slightly outside the middle ages (by around 120-150 years depending on where you put the end of the middle ages), you'd have the 30 Years' War.

The 30 Years' War (that actually was 4 seperate wars that were chained to and rooted in one another) falls into a time where more and more people started to be literate - around 10-15% of the population in the Holy Roman Empire, where the war was mostly fought, could read and write by then and not only those among the nobility. This lead to us having diaries and even "modern", semi-autobiographic novels from people who either participated or just suffered through the war, so we have a glimpse into how warfare affected the lands.

The war was, much like WW1, a new scale of warfare - especially in terms of suffering for the civilian population. Armies in this time period were mostly mercenary forces that, when pay was outstanding long enough, would just plunder - but not only out of greed, but out of necessity because they did not have a professional supply train or anything, they had to fend and supply for themselves. Generals tried to use this to their advantage and kept their armies in enemy lands as long as possible with a singular goal: bleed the enemy dry by devastating their lands. Now, if this happens one year or two years, it already leads to massive suffering - but some regions of Germany would not see the end of it for 30 years.

It is hard to put into words the suffering the people endured. Hans Heberle, a cobbler from a village near Ulm in Southern Germany, noted in 1646 (two years before the wars would end) that he was fleeing his home for the 23rd time, a number that would go up to 30 until the end of the wars in 1648. The first time he had to flee his home was in 1634 and within a year he had lost 4 of his 5 children to hunger and disease, he was noting how there were no more dogs or cats in the villages because the soldiers had caught them all for food and tells numerous tales of cannibalism.

But not only enemy armies were a problem: friendly armies that couldn't quarter in enemy territory would just as easily plunder "friendly" lands. A council member of Nuremberg noted that the City had been "besieged for three months by the enemy and then bled dry for four months by the ally".

Cities that fell to the enemy soldateska were not only plundered but thoroughly devastated - most notably Magdeburg, at the beginning of the war a metropolis by the times' standards with 35.000 inhabitants. When it was conquered by Catholic armies in 1631 in what became known as the "Wedding of Magdeburg", about 20.000 inhabitants that hadn't fled were almost completely slaughtered. After the war, some 500 people remained alive in the city.

To put it short and into plain numbers: this period of warfare reduced the population of the Holy Roman Empire by about one third. Depending on population estimates, that meant somewhere between 6 and 10 million people dead - and the vast majority of that civilians.
Edit: And to put this number even more into perspective: The war didn't rage in all of Germany, some areas were virtually untouched and even prospered (Hamburg being a notable example, as well as the region of Westphalia, which is why the peace conference was held there). So for the vast areas most affected in central Germany, this 1/3rd number is deceiving - it is much too low!

But to the point how this all ties into your question: After 30 years of gruesome warfare, people most definitely had a "let's not do that anymore" attitude. The peace treaties that were negotiated in Westphalia all aimed at producing a "lasting peace in Europe" as well as establishing a "modern" idea of a state with defined borders and agents, with the sovereign in full control and responsibility for all it's citizens and agents: responsibility also for all acts of war etc. carried out by the sovereign's subjects.

The population was elated at the end of the war, so much so that they didn't much care for who had won and lost anymore but only contemplated the suffering that it had brought. Hans Heberle notes for the end of the war: "All in all, it was such a miserable trade, that a stone would feel pity, not to talk of a human heart. Because we have been hunted like the game in the forest."

Alas, as you would probably guess, the peace was everything, but not lasting. Much like it took the world, despite all the lamentations about the horror of WW1, a mere two decades to start killing each other all over again, a few years after Westphalia, the big powers in Europe were at war again - Sweden fighting Russia in the east, Spain fighting Portugal in the West and France and Spain simply didn't even stop fighting and continued their sub-war of the 30 Years' war until 1659 - 13 years after the Peace of Westphalia.

But the Holy Roman Empire was calm for a few decades, bled out by the war but also war weary like probably never before.