For thirty years war was waged in an area where Germany and Austria are located today.
Back then, it consisted of many small countries that were ruled by their respective princes.
Since 936 AD these countries were grouped together under the term “Holy Roman Empire of the German Nation“.
Most of the time it was ruled by an emperor.
At the time of the Thirty Years’ War these were Emperor Matthias, Emperors Ferdinand II and Ferdinand III.
Since 1438 the emperors came from the Habsburg family.
The other princes’ fear of the constantly increasing power of the Habsburgs eventually was one of the causes of the Thirty Years’ War.
So the issue at stake was supremacy in Germany and Europe.
But it was also a fight between Catholics and Protestants who argued over the ”right” faith.
It is considered one of the longest and most cruel wars in history.