A war begins, and so begins the end of the European medieval period.
The medieval era had, of course, been ending for some time: Luther’s declaration, the wars of religion, the Thirty Years War and the Treaty of Westphalia, the scientific revolution—all of these tore the present away from the past and thrust its people into a new world, literally and figuratively.
Reactionaries bemoaned this newness and sought to wrench their world back; Enlightenment philosophes celebrated the demise of the old and considered the wrenching mere birth pangs for the unbounded future. Ordinary people went about their business, adjusting to spread of literacy and the advancement of capital, managing, as always, what the world presented to them.
Modernity arrived at different times in different throughout its 4-century advance: the Dutch and the Scots were the vanguard, France thrashed violently between the old and the new, Spaniards retreated, and the German-speaking peoples went in all directions in their various lands.
It is the long holdout of those German-speaking peoples and the empires they proclaimed which carried the medieval into the modern era; the Kaiser and the Emperor were the last holdouts.
And thus the war, begun by these medieval powers, brought them to their end.