The Thirty Year's War led to the loss of a third of Germany's population, the Paraguay War led by some estimates to the complete decimation of Paraguay's population by 90%. How were such large man-made losses of life possible before the invention of weapons of mass destruction?
First: a loss of population from pre-war levels is a very different thing than direct counts of casualties. The Holy Roman Empire's population was roughly two-thirds in 1650 of what it was in 1615 (although some regions suffered much more directly), but much of the population loss were simply refugees, loss of territory, and other forms of expatriation.
Of the many who did die, only a relatively small fraction would have died directly from battlefield causes. In the case of the Thirty Years' War, constant disruption to agriculture and commerce resulted in many deaths from famine and disease. This scenario also results in a population much less willing to procreate, and substantially increases infant and childhood mortality rates (and maternal mortality, for that matter).
Armies in the pre-modern world were often effectively swarms of locust: even ostensibly friendly armies, like the Lutheran Swedes in Protestant Brandenburg, would inflict grievous harm on whatever region they passed through. Simply supplying these armies could absorb a large quantity of the available food and essentials, before we even talk about looting and pillaging (or incidents like the Sack of Magdeburg, where a victorious army slaughtered ~20,000 townspeople in a single day).
In short, battlefield deaths in the Thirty Years' War were only a portion of the overall deaths resulting from the war. I expect something similar for the Paraguayan War.
My main sources are Peter Wilson's recent Europe's Tragedy: the Thirty Years War alongside Geoffrey Parker's significantly older Thirty Years' War.
It's important to note that the Paraguayan population didn't decrease overall by 90%; that was the impact to the population of fighting age males. Franciso Solano Lopez's policies essentially conscripted every possible male in Paraguay over the course of the war as the situation grew more and more desperate. There was also the massive displacement of refugees described by u/Aethelric as men fled or even joined the Triple Alliance forces to avoid conscription into certain death in Lopez's army (which, by the end of the war, included many young boys and old men).