The Thirty Years War, fought from 1618 to 1648, was largely a religious war which was fought between Catholics and Protestants and has gone down in history as one of the most devastating wars in all of European history.
Regular Websites
Encyclopaedia Britannica: Thirty Years' War
Delves into the war, the nations which fought in it as well as the reasons they did so, and how that fighting affected the nations as well as the war. - Encyclopaedia Britannica: Thirty Years' War
http://www.britannica.com/EBchecked/topic/592619/Thirty-Years-War
How Stuff Works: Thirty Years' War
Provides a details article about the Thirty Years' War, with sections which address the causes which led to the war, the course of the war, and each of the battles and revolutions. - How Stuff Works: Thirty Years' War
http://history.howstuffworks.com/european-history/thirty-years-war.htm
Contains sections on Ferdinand II, the Bohemian Alrecht von Wallenstein, the Swedish intervention, and the French and their part in the war. - The Thirty Years War
http://mars.wnec.edu/~grempel/courses/wc2/lectures/30yearswar.html
This website displays information about both of the wars which are referred to as the "Thirty Years' Wars." The section about the so-named war which lasted from 1618 to 1648 discusses the origins of the conflict, the Bohemian period, the Danish period, the Swedish period, the French period, and the Treaty of Prague. - The Thirty Years' Wars
http://www.thecaveonline.com/APEH/thirtyyearswar.html