Thirty Years' War (may 23, 1618 – may 15, 1648)
In Europe, The vast collection of German territories that made up the Holy Roman Empire had become religiously divided, with some converting to branches of Protestantism and others remaining strongly Catholic. Over in Spain, the population remained devout Catholics but were still attempting to exercise their rule over the Protestant Netherlands. As well as this, both Spain and the Holy Roman Empire were ruled by members of the Hapsburg family which put France in a vulnerable position of being sat between two Hapsburg-ruled nations. This meant that France, despite being Catholic, felt a need to exert a measure of power over the Hapsburgs to remain politically balanced.
As both religious and political tensions grew it eventually came to a head in the May of 1618. As the Holy Roman Emperor was about to die without any children, he chose a successor by the name of Ferdinand II. Ferdinand was a devout Catholic who intended to force Catholicism across the Empire, so when he sent royal representatives to the German state of Bohemia, a revolt broke out among the people. This revolt was then backed by the Nordic kingdoms of Sweden and Denmark-Norway as well as the other Protestant German territories. To fight this rebellion the Holy Roman Empire allied themselves with Spain and thus began the Thirty Years War.
Battles broke out throughout the Holy Roman Empire between the Protestant territories and the Catholic nations for over a decade with each side being supplied by various allies. Eventually however these allies began to fully join the war themselves either for religious or political reasons; the first to do this was the Kingdom of Sweden in 1630 transforming the conflict into an all-out European War.
Spain soon began to use the war as an opportunity to finally stamp out Protestantism in the Netherlands. This became the final straw for France who did not want to be completely surrounded by Hapsburgs and so joined the war in 1635, resulting in the French Catholics actually allying themselves with the Protestant Dutch. Over the next thirteen years the war would continue on with many battles fought throughout Europe, with nations such as England, Scotland, Russia, Hungry, and even the Ottomans all providing support to one side or the other.
The War finally came to an end on 15th May 1648. Over the final four years, Spain, France, Sweden, the Netherlands, and the Holy Roman Empire all signed various treaties establishing peace with one another. Spain now recognised the Dutch Republic as it's own independent nation and the German provinces of the Holy Roman Empire were each allowed to choose their own religion. Although the war had ended however, it had given rise to terrible famines and disease across Europe which killed millions. As well as this, the problems that caused the war in the first place still existed to some extent as Catholic-Protestant rivalries still remained.
The Thirty Years' War was the last major religious war in mainland Europe and changed the balance of power across the continent significantly. The Holy Roman Emperor lost much of it's influence over the German states and France and the Netherlands both became powerful nations each entering a sort of Golden Age with colonial holdings and scientific achievements increasing over the next century.
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