See also Thirty Years' War overview for greater detail.
The Thirty Years' War occurred for a number of reasons, although it is principally a religious conflict between Protestants and Catholics. The Defenestration of Prague, although relatively trivial in itself, was to become a defining moment. The will for the preservation of the Habsburg empire is also central.
The election of the Catholic zealot Ferdinand, Archduke of Styria, as King of Bohemia caused the Bohemian Protestants to fear for their religious freedom, and in May 1618, at Hradcany castle, two Catholic councillors (Martinitz[?] and Slavata[?]) were thrown from the castle windows. Soon the Bohemian conflict erupted in the entirety of Greater Bohemia, effectively Bohemia, Silesia, Lusatia and Moravia, which was already riven by conflict between Catholics and Protestants. This confrontation was to find many facets and mirrors across the continent of Europe with the involvement of France, Sweden, inter alia.
Had the Bohemian rebellion remained a purely Eastern European affair, the Thirty Years War would have been over in fewer than thirty months. However, the weakness of both Ferdinand and of the Bohemians themselves led to the spread of the war to Western Germany. Ferdinand had been compelled to call on his cousin, King Philip IV of Spain for assistance. The Bohemians had called on the Calvinist Frederick V, Elector Palatine to be their King.
The Battle of White Mountain, near Prague in 1620 was a serious blow to Protestant ambitions in the region. The rebellion effectively collapsed, and widespread confiscations of property and suppression of the pre-existing Bohemian nobility ensured that the country would return to the Catholic fold after more than a century of Hussite and other heresy. The Spanish, seeking to outflank the Dutch in preparation for the soon-to-be-renewed Eighty Years' War took Frederick's lands, the Rhine Palatinate.
Christian IV of Denmark leads an army against the Holy Roman Empire, fearing that Denmark's soveringty as a Protestant nation was being threatened. It also helped that the French regent Cardinal Richelieu was paying for it. The Danes are unsuccessful, and from 1629-1630, more land is subjugated by the Catholics.
The Swedes, lead by Gustavus Adolphus, attack the Holy Roman Empire. The Swedes also fear Catholic aggression against their Protestant country. From 1630-1634, the Swedes drive back the Catholic forces and regain much of the occupied Protestant land. In 1632 Gustavus Adolphus dies at the Battle of Lützen (although the Swedes win the battle), and in 1634 the Swedes are defeated at the battle of Nordlingen, but they continue to fight in the war until the end in 1648.
The French, although a Catholic country, opposes the Holy Roman Empire, and now enters the war at the Protestantic side. Many battles ensure, but neither side can gain a clear advantage. In 1642, Cardinal Richelieu dies. In 1643, Louis XIII[?] of France dies. Louis XIV comes to power at 4 years of age and is appointed Cardinal Mazarin as his regent. Cardinal Mazarin is interested in restoring peace to the region. In 1648, the war ends with the Peace of Westphalia.
Beacuse of the destruction endured by the Germanic people during this war, they develop a strong militant spirit to prevent future disaster such as this.