World War I and the Thirty Years War have this in common: the deaths attributable to them were caused by developments in weapons technology that outpaced developments in logistical, tactical and social technology.
In the case of World War I, armies had recently developed artillery that was able to fire huge amounts of accurate, deadly shells, weapons far more effective than any artillery that had been seen before. These weapons accounted for the greater part of the casualties in the major battles. Machine guns accounted for another great portion of the casualties, when came to playing defense as opposed to offense. One of the big reasons that the slaughter of troops was so great was that they were marching step by step into the fields of fire of these weapons, without the ability of generals to call them back or execute more precise manuvers...because the generals couldn't easily communicate with anyone on the front line. Radios were a thing at the time, but portable radios for soldiers hadn't been invented yet. So your ability to do tactics remained basically at the Napoleonic level while your weapons had the ability to mow those Napoleonic formations down in rows.
The situation with the Thirty Years War is similar. Not in the sense of the weapons themselves being more deadly...but simply being more easy to use, which meant that you could field much larger armies...who were still operating on the mercenary model...which meant that they had to live by foraging...despite being armies bigger than the poor peasants were used to feeding. Essentially, the rapid development in weapons technology outpaced anyone's willingness to change how armies were supplied, and it wasn't until 1/3 of Germany starved from having to feed the roaming mercenary armies that people thought, maybe we ought to have actual supply lines for these soldiers instead of expecting Hans Blumenstein Of Waggelsdorf to feed every soldier that barges into his house.
Beware wars with excellent guns and outdated logistics. Their efforts against each other will be even less pretty than your usual war.