The Thirty Years' War refers to a religious and political conflict that lasted from 1618-1648. The war involved most of mainland Europe in tumultuous battles and resulted in financial and resource devastation throughout most of the participant states. The major consequence of the Thirty Years' War was the destabilization of the formerly powerful Holy Roman Empire, while the major causes of the war are a complex network of treaty failure, imperialistic designs, and religious conflict that can be said to be the culmination of several centuries worth of strife in Europe.
With the staggering rise in popularity of Martin Luther a century earlier, the hundreds of states in the Holy Roman Empire, which were largely German, had to contend with vicious wrangling between Catholic and Lutheran followers. To staunch this seemingly endless internal conflict, The Holy Roman Emperor Charles V signed a treaty known as the Peace of Augsburg in 1555, allowing each of the German states the right to practice Lutheranism or Catholicism as they decreed. However, the rise of other religions, such as Calvinism, were not permitted by the Peace of Augsburg, thus leading to renewed religious conflict amongst the states by the turn of the 17th century. The attempted but failed overthrow of the Catholic leader, Ferdinand II, of the German state of Bohemia in 1618 is usually cited as t he first official event in the Thirty Years' War.
In 1625, Denmark became involved in the conflict by lending support to the Bohemian rebels against Ferdinand II of Bohemia. In this segment of the war, Ferdinand eventually secured victories over the Danish funded rebels, gaining a second victory for the Catholic front. Denmark's humbled king swore to stay out of the conflict in the future, but the rising tide of Catholic victories had now begun to seriously trouble Protestant leaders throughout Europe. France and Sweden, both heavily Protestant countries, signed accords of cooperation in 1630 and joined forces to defeat the Catholic rulers of the Holy Roman Empire. Sweden was roundly defeated, leading to the Treaty of Prague, which was meant to restore some sense of order to the continent, but this was undermined by the plans of France.
The final period of the Thirty Years' War was marked by an all-out assault by France against the Holy Roman Empire, as well as France's ancient enemy, Spain. The conflict might have continued for many more years, but the mounting cost and death toll were compounded by the deaths of most of the principle players by the mid-17th century. In 1648, with all sides nearly exhausted, the Peace of Westphalia was signed, bringing peace to much of the continent, though conflicts between some nations continued.
The treaty that ended the Thirty Years' War granted more rights and independence to the many states of the Holy Roman Empire, thus destabilizing the Catholic central government of the region beyond repair. In addition, France and Sweden annexed several areas under German control. Pestilence and battles led to astounding casualty rates for both civilians and soldiers, with some historical scholars suggesting that the German regions may have lost up to 20% of the population due to war-related deaths.