This agglomeration which was called and which still calls itself the Holy Roman Empire was neither holy, nor Roman, nor an empire.- Voltaire
In other words, the title of this whole thing is very misleading. And yet, in order to understand the causes of the 30 Years War, it’s necessary to have at least a basic grasp of the nature and structure of the Holy Roman Empire.
Back in 800 AD, the Frankish king Charlemagne was proclaimed Emperor by Pope Leo III, the first since the end of the Roman Empire some 400 years before. The title died out with Charlemagne’s line, but was revived in 962 by Otto I. This empire was holy only insofar as it was officially sanctioned by the pope, only Roman because its emperors saw themselves as inheritors of the Roman tradition, and not so much an empire as a loose collection of states of varying sizes.
The empire was centered in Germany and Austria, and even though the Emperor was usually descended from an ongoing dynasty- most notably, the Habsburgs- he (and sometimes she) were always elected by seven German princes known as electors. These were:
The Duke of Saxony
The Margrave of Brandenburg
The King of Bavaria
The Count Palatine of the Rhine
The Archbishops of Mainz, Trier and Cologne
These guys controlled huge amounts of German territory, but the Emperor controlled much of Austria himself as a kind of feudal lord. In addition, he ruled Bohemia, Moravia, Silesia and Lusatia, and depending upon the time period, parts of Hungary, Switzerland, Italy and Yugoslavia. Eventually, the Habsburgs also became the kings of Spain, which greatly expanded their influence although Spain tended to put its own interests first, Habsburg or not.
On paper, the Holy Roman Empire covered most of Central Europe, but in practice, the Emperor was herding cats. The Electors were not the most obliging bunch, and under them were hordes of lesser princes who were always squabbling over their piece of the power-pie.
That’s why there are so many castle ruins in Germany. Every guy with a title and two acres to call his own had to have one. That’s also why so much European royalty is mostly German *coughHouseofWindosrmyasscough* No matter how hard up you were, or how British your smile, you could always find an impoverished German aristocrat to take you on.
The general rule was that, the further north you went, the less power the emperor had. This was all exacerbated by the Protestant Reformation. In many ways as political as it was religious, the Reformation provided an opportunity for restless German princes to gain some independence from the empire.
It’s probably not too surprising that the Reformation first took hold in the North (Saxony), and even though it spread all over the empire, its lasting gains remained in Northern Europe. Crucially, three of the four secular electors were Protestant at the start of the 30 Years War.
Frederick of the Rhineland-Palatine was Calvinist, John George of Saxony was Lutheran (and quite anti-Calvinist, just to make things interesting), John Sigismund of Brandenburg was Calvinist, although a cautious one, since everyone else in Brandenburg was Lutheran, and Maximilian of Bavaria was Catholic. Maximilian however, was highly pro-German, and was not above allying himself with Protestants if it helped take the Habsburgs down a peg.
Even though the archbishop-electors were Catholic, they weren’t always automatically loyal to the Habsburgs either. Needless to say, all of this decentralization created a highly volatile situation by 1618. The Emperor had very little direct power, religious unity was completely lacking, and the problems started by the Reformation were exacerbated by the Peace of Augsburg and a number of minor conflicts in the early 17th century. That all of this led to a major war engulfing the whole region isn’t at all surprising.