The early stages of the Thirty Years War (1618-31)

Opening phases of the war (1618-35)

Many people figured war would start in 1621 when a truce between the Dutch Republic and Spain was due to expire.  In fact, it started in Bohemia (the modern Czech Republic) and Germany over the succession to the crown of the Holy Roman Empire that the Hapsburgs had held for generations.  However, there was no guarantee the electors would choose another Hapsburg when the old emperor Matthias died.  Since six of the seven imperial electoral votes might easily be split between three Catholic and three Protestant electors, the Bohemian king's electoral vote could be the decisive one.
Here was where the trouble began, because Ferdinand of Styria, the king of Bohemia and heir apparent to Matthias, was an ardent Catholic, whereas Bohemia had been a hotbed of religious turmoil ever since the Hussite revolt in the early 1400's.  When Ferdinand tried to end the Protestant Bohemians' religious freedom, they retaliated by defenestrating (throwing out a window) two imperial ministers in the famous Defenestration of Prague (1618) and deposing Ferdinand as their king.  Although the ministers miraculously survived the sixty-foot fall, the peace did not survive with them as the turmoil quickly spread across Germany.
Unfortunately for the Bohemians, when they rebelled against Austria, they elected a mediocre king, Frederick of the Palatinate, who only brought moral support from other Protestant powers.  Cossack raids stirred up by Poland diverted the one bit of substantial help they might have gotten, troops from Transylvania.  Meanwhile, Spain, Bavaria (as head of the German Catholic League), and the Pope were helping Austria with men and money.  Consequently, the Bohemian War (1618-22) was not much of a struggle as Ferdinand (who had since become emperor) easily swept away Bohemian opposition.  Ferdinand and his followers confiscated large tracts of land, exiled Protestants, and reclaimed Bohemia for the Catholic Church.
However, growing fear of a resurgent Hapsburg dominance stirred up activity across Europe in two main theaters of war, one aimed against Spain and the other against Austria.  First of all, hostilities between Spain and the Dutch Republic resumed as expected in 1621 when their truce ran out.  England also declared war on Spain in 1625 and joined the Dutch in a raid on Cadiz that ended in an embarrassing defeat for the Dutch and English.  After this, England became more involved in its own religious and political squabbles that culminated in civil war in the 1640's.  This kept them from playing any major role in the wider conflict unfolding on the continent.
Meanwhile, France was also active, fighting Spain over strategic towns and passes in Italy.  If the French could control this area, they could block the flow of Spanish troops to the Netherlands along the so-called Spanish Road.  However, France's effort was somewhat ineffective at this point, largely because of turmoil at court.  Cardinal Richelieu, who wanted to commit France wholeheartedly to fight the Hapsburgs, had to fight for his own political life against the Queen mother, Marie de Medici.  Richelieu and his policy would eventually triumph, throwing the full weight of France against the Hapsburgs with momentous results for European history.  But for now, France's effort was of little account, and Spain held on in Italy.
Despite these victories, ten years of warfare were taking their toll on Spain's wealth, manpower, and ability to protect its treasure fleet, which the Dutch captured for the first time in 1629.  This and Spain's already seriously damaged finances forced it to declare bankruptcy, leaving outstanding loans unpaid.
Meanwhile, the Austrian Hapsburgs' overwhelming victory in Bohemia had led Denmark to invade Germany in 1625 supposedly in defense of Protestant liberties.  The Hapsburg general, Albrecht von Wallenstein, and the Catholic League's general, Tilly, made short work of the Danes, thus winning what is known as the Danish phase of the war (1625-29).  The emperor Ferdinand felt so strong after his victory that he issued the Edict of Restitution in 1630.  This declared that all land taken from the Catholic Church since 1555 must be returned to the Church.  The Edict of Restitution drove thousands of Protestants from their homes and aggravated an already turbulent situation. It also alarmed and angered German princes, Catholic and Protestant alike, who felt the emperor was overstepping his constitutional powers.