This is an interesting and complicated question. If I understand it correctly, you’re wondering how the role of the “fight to the death” mentality of the Paraguayans and the “kill all the Paraguayans” mentality of the Allies contributed to the enormous loss of life in the war, why these mentalities existed, and which one was more significant. Did I get that right? Overall, I would argue that it was a combination of the two, not one above the other, that caused the high death toll. I find the two to be inextricably linked.

In regards to the “to the bitter end” mentality, Paraguayans were noted worldwide for their epic defense of their nation despite overwhelming odds. What inspired this commitment? First, López was a dictator. He mobilized the population by force. Most of the male population was drafted during the first half of the war. Meanwhile, he crushed dissent, imprisoned and banished rivals, forced the elites to comply with his agenda, and used his secret police to maintain order. He is even described by some as a madman, deluded by notions of being South America’s Napoleon. However, I find this interpretation to be incomplete; force alone doesn’t account for why so much of the population was willing to and did give their lives for the country.

In addition to forced compliance, the Paraguayan press played a major role in stirring up resistance to the Allies. They published stories in both Spanish and Guaraní that emphasized patriotism, duty, nationalism, and strength. They also created a sense of danger to la patria and el pueblo at the hands of the bloodthirsty allies. These messages played well in Paraguayan society which really did fear for its existence. Paraguay had a long history of border conflicts with Brazil, and Argentina still considered Paraguay a rogue province that belonged to Buenos Aires. The common Paraguayan fought to the bitter end because they were fighting for their homes, their families, their land, and their existence. Commoners viewed the Allies as invaders who sought to destroy Paraguay itself. This was especially true after the terms of the Secret Treaty between the Allies became known, which Paraguayans interpreted as proof that foreigners planned to destroy the government, the state, and el pueblo. Therefore, there was a huge swath of the Paraguayan population who committed themselves to giving their lives. They fought bravely because everything was on the line. One Paraguayan soldier wrote in 1869: “I am here to fight and if necessary to die, but never to surrender.” When a commander was asked why the women and children won’t surrender, the Paraguayan commander explained: “They are here to defend la patria. They will surrender when there is no one left to defend it.”

Finally, it is worth noting that not all Paraguayans were willing to fight to the bitter end. Many indigenous people fled into the sparsely populated borderlands outside of state control. Additionally, a large number of Paraguayans joined the Allies, forming a large contingent within the Allied armies that saw action throughout the war. Political refugees fled Paraguay if they got the chance. Thus, devotion to the cause was not total.

The Allies also contributed to the enormous death toll. The War of the Triple Alliance is considered the only instance of modern total war in Latin American history. Like in other total wars, the violence was far reaching. Cities were leveled; farms destroyed, no prisoners were taken; massacres were commonplace (on both sides). A famous example of Allied atrocities took place at Piribebuy, when Brazilian troops rounded up the inhabitants of the town, shot many of them, locked the rest in the church, and set it on fire. Many other examples abound of Allied troops committing crimes against Paraguayans (which in turn reinforced the Paraguayans’ beliefs that the Allies intended to destroy Paraguay itself). Further heightening animosity was the propaganda that flowed into Allied countries. Newspapers exaggerated reports of atrocities committed by the Paraguayans, which caused the allied public to push for further escalations. Though they may not have supported the war completely, many allied citizens wanted López held accountable for the atrocities they read about (real and imaginary). They were willing to tolerate the war until that end was achieved.

Here too, it is worth noting that not all Allies should be considered bloodthirsty. Violent acts committed in circumstances of total war by some soldiers should not represent the actions of all allied troops. Furthermore, the war was wildly unpopular in Argentina. Argentine forces were pretty much removed from the field and sent home to extinguish rebellions in the Provinces. Many Argentine soldiers did not want to be on the front at all, having been drafted unlawfully in many cases. Brazil’s government faced anti-war sentiments especially because the public expected the conflict to be short.

To finish up, there was another major cause of death that you didn’t mention: disease and starvation. Paraguayan food supplies dwindled as more and more labor was committed to the military and as Paraguay lost access to foreign trade. Additionally, massive outbreaks of cholera swept through Paraguay, especially Asunción, endangering a population already suffering from total war and a shrinking food supply. Starvation and disease killed more troops on the frontlines than the fighting itself. It was also an indiscriminate killer; allied troops were also ravaged by disease, which contributed to drawn out nature of the conflict.

So in the end, all three of these contributed to the high rate of death. Ultimately, the mobilization and devotion of both sides, the nature of total war, and disease/starvation together caused such devastation.