A large number of casualties were civilian. Many of the forces involved, especially in the latter two-thirds of the war, weren't fighting in their own territory or even with their own forces (for example, the Swedish cavalry was largely Swedish/Finnish but the infantry were "mostly Scots and Germans").^1 Although the battleground was Germany and Bohemia, the German contingent prevalent from 1618-1620 and the early 1620s were soon overtaken politically by the Danish (1625), the Swedish (1630), the French (overtly 1635, sneakily from as early as 1618) and the Spanish (from the start). These foreign parties didn't care so much about the state of Germany as they did the geopolitical situation, particularly their own gain. Therefore looting was a common practice; it allowed states to remain at war with mercenary armies for decades straight at the cost only of foreigners', usually enemies', civilian populations. On top of that, many mercenary armies were undisciplined any many commanders didn’t really care. Many civilians tried to feed themselves by joining in the plunder as well, worsening the situation.

Although this looting often did not involve straight-up murder, it did sometimes. This was most notable at Magdeburg when, in May 1631, an imperial army escaped the control of General Tilly and looted, raped, and killed at will. During the sack the city caught fire somehow and the city, which was one of the largest in Germany, was effectively destroyed. On top of the 20,000-some civilians and defenders who died the city's grain was either stolen or lost in fire and so survivor's were left with little food and little shelter, meaning that casualties could be much larger.^2

Other than Magdeburg, looting usually comprised the illegal seizure of food and money, something which was not punished because there was never a single authority recognised throughout the entirety of the Holy Roman Empire. This seizure of food to feed armies left many regions starving, not only in the countryside but in towns in which armies were garrisoned. On top of that, the concentration of people in cities worsened by the income of armies, especially armies who had come from far away, and the general amount of travel in Germany at that time allowed the plague to blossom. In 1625, before the sack, Wallenstein brought the plague to Magdeburg/Halberstadt, killing up to 40% of the population.^3

Another reason that Germany was left so unpopulated was because of emigration. If foreign armies are trampling over your crops and executing your neighbours, you'd want to leave before the other side comes along and does the same. Not all casualties were deaths. Unfortunately this is hard to track because of the lawlessness of some regions of Germany at the time.

When comparing this war with Napoleons’, we must remember scope. The armies of Napoleon and his enemies were much larger than the ones of the 17th century, the former being in the hundreds of thousands and the latter usually the tens, but the napoleonic wars were relatively short and studded with breaks. Although conflict was not entirely continuous from 1618 to 1648 it rarely let up for as the Napoleonic wars did, never actually reaching complete peace such as happened after the Treaty of Amiens. Battles never reached the scale of Leipzig (1813) or Borodino but there were so many more that it makes sense that casualties would be so large. On top of that, we see a shift in tactics, from aggressive to defensive, in this time period. A good example is to compare the battles of Breitenfeld and Lützen; in the first, the Imperial tercios were decimated by Swedish combined arms. In the second, the imperial forces took a much more aggressive approach, reforming the defensive tercio into something a bit more capable. The use of new military techniques and strategies resulted in battles in which one side was overwhelmed or confused by the other. Battles were bloodier due to the transition. Although Napoleon did similarly with his corps he usually had one or two setpiece battles and then made peace, one example being the War of the Third Coalition, effectively ended after Ulm and Austerlitz. In the Thirty Years War, the stakes were too high to simply make peace. People were fighting to reassert the Imperial constitution; to beat back Protestantism or Catholicism; to undo the Hapsburg hegemony creeping over western Europe; to put their nation on the map as a Great Power. Nobody was willing to let up until everyone was too exhausted to keep fighting, and then some.

It’s hard to say how accurate one source’s figures are over another’s because of both how protracted the war was and the lack of a single authority to account for the damages, meaning that estimates have to be made loosely. However, the concept of the war as all-destructive is something historians tend to disagree with. Wedgwood says that Germany, excluding Netherlands and Bohemia, went from 21 million to 13 million.^4 Wilson wrote that the entirety of the Austrian Hapsburg lands went from 7.14 million to 6.465 million from 1600 to 1650 and that some of the worst-affected areas were Lorraine (60% population loss) and Wurtemburg (57%).^5 Gardiner wrote that a section of Thuringia fell from 1,717 individual houses to 627.^6 Civilian devastation far outweighed the loss of life in battles, at least when compared to similar figures from the Napoleonic Wars, in which armies lived off the land but did not loot so directly or vigorously as in the Thirty Years’ War. To this extent, although mercenary men died, civilians died in roughly equal numbers of men and women, due to indiscriminate killers that were famine or plague. In fact the devastation of this period was sensational to contemporaries and led to relative moderation in subsequent wars such as the English Civil War, or at least an attempt not to repeat the horrors which struck Germany. Although estimates may be lower than given the fact that we aren’t sure is worrying as it shows just how chaotic the suffering was.

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^1 C. V. Wedgwood, The Thirty Years War, 2005 edition, p. 265
^2 P. H. Wilson, The Thirty Years War: Europe's Tragedy, 2011 edition, p. 470
^3 Wilson, ibid, p. 782
^4 Wedgwood, op cit., p. 496
^5 Wilson, op cit., p. 788
^6 S. R. Gardiner, The Thirty Years’ War 1618-1648, 3.2