Being a cousin of the Emperors Rudolf II and Matthias, he was not expected to become Emperor himself. At the age of twenty-one he married Maria Anna of Bavaria and by her fathered seven children, but Maria Anna died in 1616 aged only forty-one. In 1621 he married Eleonora Gonzaga but this marriage remained childless.
In 1618 Ferdinand, having become the obvious heir, was proclaimed King of Bohemia and Hungary. As king he wanted to assert his power and enforce the Catholic religion by putting and end to all heresies. The Protestants of Bohemia felt protected by the 'Letter of Majesty' of Emperor Rudolf II; but when it became clear that Ferdinand would not tolerate their religion, they revolted. Ferdinand sent his stronghold in Prague. However, they were set upon by a crowd of representatives, Martinic, Slavata and a secretary, to the Protestant
Bohemian deputies and thrown out of a high first-floor window of Hradcany Palace. As they clung to the window frame, they called on the Virgin for help. "Now let your Mary help you!" cried one of the rebels as they fell. A moment later, looking down and seeing them still alive and limping away, he exclaimed: "By God, his Mary has helped!" They had landed on a rubbish-heap which had broken their fall.
In July 1618 two Imperial armies moved into Bohemia to seize Prague. With the exception of the Elector Palatine, the Protestant princes stood by without giving any support. Friedrich V came forward as the champion of the rebel cause and employed Count Mansfeld with his mercenary army. This prevented the Imperial army from taking Prague.
On 20 March 1619 the old and decrepit Matthias died and Ferdinand II was now to become Emperor as well as being King of Bohemia and Hungary. At first Ferdinand II offered remarkable concessions but, as the rebels distrusted him, they refused to capitulate.
As there was in fact no Emperor as yet, most of the Austrian provinces came into revolt. Count Thurn with his unpaid Protestant army moved on to Vienna. Ferdinand II held out, even against a deputation of the Austrian Estates pressing him to yield to the rebels' demands. However, in his courage and serenity he refused to consider giving way. At the moment of his greatest need, his brother,
Leopold of Tyrol, sent him an army which relieved Vienna.
Ferdinand II was then elected Emperor only two days before the Elector Palatine, Friedrich V, accepted the Bohemian rebels' offer of Ferdinand's crown. Ferdinand II now embarked on his terrible crusade and, on 8 November 1620, his General Tilly defeated the flower of Bohemian Protestants. Friedrich V then lost his own territory which was devastated by the Imperial army.
Even though the Spanish Habsburgs had been the head of their family and the driving force behind Catholicism, Spain's loss of military power shifted this position to Vienna, and Ferdinand II continued his crusade to re-conquer the whole of Germany for the Catholic church.
This destroyed the commerce of the independent free cities and put back the development of Bohemia. The end result of his efforts was a divided Germany with a Catholic south and a Protestant north. The properties of the defeated Protestants in Bohemia had to be disposed of. In need of cash, Ferdinand II was unable to hold them for himself, and so sold them at enormously reduced prices. This was the beginning of the immensely wealthy, powerful and almost independent nobility in Bohemia and Moravia. One of the most powerful nobles was Albrecht von Wallenstein who, when suspected of seeking the Bohemian crown, fell into disgrace. However, when Gustavus Adolphus of Sweden became a threat, it was Wallenstein with his own army who was able to halt this
menace.
The King of Spain, still the senior Habsburg, disregarded the Swedish threat and demanded that Wallenstein with his army move to Italy to fight the French. Wallenstein refused as he needed his army to prevent the Swedes from landing. However, with Imperial pressure forcing him to go, the Swedes invaded the Baltic coast, moving south to the Rhineland provinces and deep into Bavaria, then across Austria itself.
Brandenburg and Saxony then declared themselves to be with the King of Sweden. In May 1631 Tilly's troops sacked the city of Magdeburg with such savagery that it remained a stain on both his and Ferdinand II's reputations. In September 1631 Tilly was defeated at Breitenfeld and again, in April 1632, at the river Lech, where Tilly was fatally wounded. 
Wallenstein was now recalled and in November 1632 was defeated at Luetzen. However, as the Swedish king had been killed in this encounter, Wallenstein became the supreme figure on the battle-field. In 1632, after twenty-four years of war, the original reasons for the war had been forgotten, and all parties were engaged in it merely for their own gains. The German princes, Catholic and Protestant, aimed at territorial gains. The French wanted to destroy the power of Spain as well as gaining German territory. Wallenstein began to act independently but was murdered in 1634. When Ferdinand II died in 1637, instead of conquest all his army could do was preserve the Austrian inheritance and support Spain.

Source: Leo van de Pas