The Thirty Years War was a devastating religious war in Europe in the early seventeenth century. It was the bloodiest war in Europe until World War II, with as much as one third of the population killed and hideous atrocities.
The war was the result of increasing tensions in the Holy Roman Empire between Protestants and Catholics. Attempts had been made to negotiate settlements, but the Protestant side had only been represented by Lutherans. Calvinists did not consider themselves bound by the terms, and problems ensued. In 1608, a series of Protestant states (that is states within the HRE) formed the Protestant Union. The Catholic states returned the favor by forming the Catholic League in 1609. Each of the two groups was determined to stop the other from territorial gains.
The fuse that began the war was a strange situation known as the "Defenestration of Prague," when at a meeting to protest government action, several Protestant officials seized two Hapsburg (and therefore Catholic) officials and threw them out a window into the moat seventy feet below. They were only saved because the moat was also a latrine dump.
The war is typically divided into four phases: Bohemian, Danish, Swedish and French. The distinction is to complicated to explain here; but the links below can provide more detail. By the last (French) phase, the war was no longer over religion, but over territory. Throughout the war, there were numerous atrocities, such as the following description written by an eye witness:
T]he soldiers stretched out a hired man on the ground, stuck a wooden wedge in his mouth to keep it open, and emptied a milk pail full of stinking manure droppings down his throat—they called it a Swedish Cocktail. Then they used thumb-screw which they cleverly made out of pistols, to torture the peasants….They put one of the captured bumpkins in the bake oven and set fire in it…I cant say much about the captured wives, hired girls and daughters because the soldiers did not let me catch their doings. But I do remember hearing heir pitiful screams in various dark corners.
The war ended with the Peace of Westphalia which among other things, left France the strongest country in Europe. I urge you to read the links below for more information.