What made the Paraguayan War so deadly compared to other conflicts of the time?

Hey, I’ve written on this topic before; /u/OldWorldGlory found some of my past answers, nice! The first one he or she listed walks you through some major reasons why the casualties were so high. That response would be a good place to start. I’ll add more information specific to your question as well:

First, the numbers you read on Wikipedia are hotly debated. Unfortunately, we do not know Paraguay’s exact population before or after the war, so we can’t say for sure how many people died. There are a few censuses taken here and there during the 1800s, but these did not precisely count the population. I’ve seen studies where historians try to reach an accurate estimate by adding between 15% and 37.5% onto these censuses to account for underrepresented rural indigenous and pardo populations. That’s a pretty significant increase! The 60% to 90% you saw on Wikipedia is the estimate of the loss of male fighting age population, but here too, we do not know exactly the percentage. As for total loss of population, there are estimates anywhere from 20% to 75%. As you can see, historians are all over the board on these estimates because they are making due with inadequate sources. Regardless, the exact numbers don’t really matter that much; the war was absolutely devastating, traumatizing, and change-inducing for Paraguay.

As was usually the case in many devastating pre-industrial conflicts, the biggest killers during the Paraguayan War were disease and starvation. In the first year of the conflict alone, Paraguay lost 6,000 men to disease. Three years into the conflict, observers estimated Paraguay had lost 50,000 (probably an overestimate) to disease. Soldiers died of dysentery, smallpox, scurvy, measles, pneumonia, and cholera according to historian Jerry Cooney. This was made worse by the fact that as the war dragged on, more and more men and women were sent to the front, which meant fewer able bodied people were at home to produce food. Rationing and food shortages increased both civilian and military susceptibility to disease, which in turn required more people to be pulled from home to the front to replace those who had been killed or had died. Thus, disease and starvation caused a cyclical demographic collapse, accounting for most of the casualties. Furthermore, towards the end of the war, there was a massive cholera outbreak in Asunción and in other Paraguayan communities. The outbreak killed thousands more and caused even greater hardship on those still able to contribute to the war effort. To make matters still worse, after the fall of Humaitá, many Paraguayans fled or were forced from their homes and farmland, further disrupting the economy and increasing the death rates from disease and starvation. That’s not to say that the fighting wasn’t devastating. It was! The war was absolutely vicious, consuming a huge number of soldiers. It was South America’s only total war, and disease made the conflict so much worse.

Historians also debate why the war dragged on for so long. First, there was the determined (some say crazy) Paraguayan leadership which refused to accept defeat. In some ways, it makes sense that López did not want to surrender. The Allies had secretly agreed that they would not accept surrender unless López was removed from power. Because of this, the leadership did not accept peace overtures, even though the war was clearly over. Secondly, despite their massive numerical advantage, the Allies struggled to mount effective campaigns early in the war and often failed to keep their forces supplied. The conflict bogged down into trench warfare and sieges. Allied forces also suffered outbreaks of disease, which reduced their military potency. Most pertinent for Paraguay is the fact that the press, popular culture, women, upper classes, and the military all portrayed the war as a patriotic and crucial fight that had to be won at all costs. The war became about doing one's duty for the country and the family. As defeat became more and more likely, Paraguayans began to feel that they were fighting for the survival of their families and their way of life. Many felt that the Allies would exterminate all Paraguayans, so it became a war “to the bitter end.” Not everyone felt this way; many Paraguayans fled or deserted. There was even a sizeable contingent of Paraguayan soldiers fighting in the Allied armies. But a huge amount of the population fought on and on and on despite the casualties.

Based on my knowledge of the American Civil War (I am not an expert, so other users should feel free to chime in), the Confederates did not have an apocalyptic mentality like the Paraguayans developed. Though devastated, their leaders accepted defeat much sooner. The American South is also much larger in terms of area, so the destruction wrought by the armies was not as concentrated as it was in tiny Paraguay. Finally, Paraguay’s population was significantly smaller than the CSA’s, so casualties from fighting and disease were much more dire for Paraguay.

Edit: Thank you!