It has been famously pointed out that the Holy Roman Empire was neither holy, nor Roman, but it was also not an empire in the sense people expect when hearing the term. In theory, the emperor was the highest prince in Christendom, and his dominion extended the length and breadth of Western Europe. The empire had been created by the papacy in 801 when Pope Leo III famously crowned the supposedly unwitting Charlemagne in Saint Peter’s Basilica, intending to recreate the Western Roman Empire. In truth, the imperial power did not extend beyond central Europe, which by the beginning of the 16th century included Germany, northern Italy, and the Netherlands. Even in these lands, however, the emperor struggled to command obedience. His dominion over northern Italy was theoretical only, the cities of the Netherlands were deeply conscious of their ancient rights and privileges, and Germany had long ceased to be compliant. The latter had become a collection of principalities, dukedoms and bishoprics which vied with each other and pursued their own agendas. They were, however, united in only one sense: denying the emperor power and resisting attempts to centralize the government.

The Thirty Years' War was one of the most horrific conflicts in history, and though it is widely viewed as a religious struggle, that was only part of the complicated war. Calvinists and Lutherans did not get along, and both persecuted some of the more radical Anabaptist sects. At the same time, one major motivation behind the war was Holy Roman Emperor Ferdinand II’s determination to rule all of the empire and not be just a figurehead. There were struggles between rulers and their estates over power, and Catholic France later entered the war on the side of the Protestants in order to counter the Habsburgs’ power.

The Battle of Breitenfeld, fought in September 1631, was the first major Protestant victory and widely considered the crowning achievement of Swedish King Gustavus Adolphus’ military career. Through his establishment of communication and supply lines at strategic points across the Baltic Sea, the securing of Protestant alliances, and his use of combined arms, amongst his other trademark techniques, the Swedish forces, against all odds, defeated their rivals. 6,000 or so Catholic soldiers were captured, many of them later incorporated into the Protestant forces. Whatever remained of the survivors vanished into the dark of the night.

On November 16, 1632, the king led a Protestant army of Swedes and German allies against Wallenstein’s army near Lützen in Saxony, and during a hard-fought Protestant victory that forced Wallenstein to retreat, Gustavus was killed while leading a cavalry charge. Count Gustavus Horn, who had much to do with the Swedes’ success at Breitenfeld, took command of the Swedish forces, who remained an important factor in the war until its end in 1648 but were no longer dominant on the battlefield. At the same time, the novel Swedish formations were quickly adapted by Wallenstein and other commanders, who increased the number of muskets, arquebuses, and light artillery integrated within the battalion formation.

Gustavus’s integration of pike, shot, cavalry, and field artillery is often seen as the first demonstration of the concept of “combined arms,” and more than one historian considers some of the battles he led in the Thirty Years’ War to be the first great modern battles. That said, the Catholic armies often fought Gustavus’s Swedes to a virtual standstill, and the Swedes’ sense of superiority was lost upon his death, indicating the crucial factor may have been Gustavus himself, due to his charisma and tactical skill more than his military innovations. Moreover, it’s possible the sheer destruction of the conflict would have compelled the armies to modernize anyway.