In the beginning of the 17th century, war was essentially unlimited. The Thirty Years War was pretty much the examplar of how bad things could get if you combined the size and effectiveness of an Early Modern army with the Vegetian style of warfare that had previously been employed by fairly small, disorganized Medieval armies. Germany was extremely devastated by the Thirty Years War. About a third of Germans died, with some states suffering much worse losses.

After the Peace of Westphalia, there was a change in how war was conducted within Europe. Rules made wholesale destruction and pillaging significantly less common. In The Art of War in the Western World, Archer Jones writes:

The new way of supply and is often scrupulous care for civilian property [in the late 17th century] did reduce the needless costs of war and in this respect limited it. Sometimes armies carried the circumscriptions on the conduct of soldiers to ridiculous extremes. For example, a small French force, fleeing from pursuing Austrians, crossed a river on a ferry, duly paying the civilian ferryman for his services. When the Austrians reached the ferry, they abandoned their pursuit because they lacked the money to pay the ferryman. Of course, in this instance, the limitation on the conduct of the soldiers did not alter the outcome; if the retreating French had disregarded the sanctity of civilian property and sunk the ferry, the Austrians still could not have crossed regardless of whether they were willing to seize the ferry.

So the 17th century was a time of transition, when warfare possibly reached an apogee of brutality and devastation, which in turn lead to a kind of reform which lead to significantly more limited warfare.

As for casualty rates, interestingly, over the course of the 17th century, the casualties among the victors stayed fairly stable, but the defeated suffered fewer casualties, perhaps illustrating the declining effectiveness of pursuing routed foes, according to Jones:

Casualties of the victors during the Thirty Years' War amounted to 12.5 percent of their forces, those of the defeated, 37.4 percent. These percentages differ from those of a hundred years earlier, but warrant no conclusion because the data lack comparability.

But comparable data for the period 1649 through 1701 indicate a marked change. Victor's casualties, 12.6 percent, did not change, but the losses of the defeated dropped to 27.6 percent. Although the linear system became universal and many soldiers had plug bayonets during the latter part of this period, permanent regiments and drilled standing armies seem the best hypothesis for explaining most of this dramatic decline. Disciplined, permanently established armies could better withstand the disorganization of defeat than could the often poorly trained mercenary infantry of the Thirty Years' War.