A part of history I always found interesting was the 30 Years War.
In 1594, Henry IV was coronated king of France. But France was Catholic, a d he was very protestant. Ultimately, he converted but he put forth a policy of being permissive to protestants in France. After his death, his child, Louis XIII takes the reign. He, a long with his cardinal, are Catholic but very wary of the Habsburgs, the ruling family of Austria-Hungary and Spain. So when the a war broke out between protestants and Catholics, Louis XIII sides with the protestants in a move his cardinal called 'raison d'etat' or reason of state.
This, in a way, invents the concept of a political state separate from bloodlines and religion in Europe, and leads the way for Louis XIII's absolute monarchy.