The Catholic Holy Roman Emperor Rudolf II intervened in the War of the Jülich succession by occupying the small, but strategically located, protestant United Duchies of Jülich-Cleves-Berg it seemed that would be the fuse to light the keg.
Ever since the reformation the German states had been a powder keg. Lutheranism was widespread, especially in the Northern states, while Calvinism had also gained a foothold, but many of the rulers as well as large parts of the population remained catholic. The Treaty of Augsburg had made Lutheranism legal, but had not resolved the underlying problems as it tied the religion of each territory to the religion of it's ruler.
A combined French-Dutch army was prepared to invade the United Duchies and oust Rudolf, who appealed to his kinsman Philip the Third the emperor of Spain. Philip, who had arranged a truce with the Dutch a year before reluctantly agreed that troops from the Southern Netherlands would come to come to Rudolf's aid.
In the event the invasion of the United Duchies was called off, because the French King Henry IV was assassinated in Paris.
In the meantime Rudolf had been increasingly marginalized by his own brother Matthias who, in the wake of the long and unsuccessful war against the Turks, had forced him to cede Austria and Hungary.
Seeking to maintain at least the Kingdom Bohemia, as well as his now largely ineffectual title as Holy Roman Emperor, Rudolf nominated Archduke Ferdinand II of Austria as his heir. Hoping that Spanish backing would prevent Matthias from making a further move against him.
Apart from making many of the Habsburgs, with the possible exception of Ferdinand, unhappy the news also led to riots in Prague. When Rudolf used his army to suppress the riots his brother intervened and had him locked up and forced him to abdicate.
When Ferdinand sent his envoys to Prague they were however badly received, in fact they were thrown out the window (the so-called defenestration of Prague). Although still smarting from the slight by his brother Matthias was appalled by the insult to royal power and marched on Bohemia suppressing the revolt and installing Ferdinand as king.
The conflict served to further harden the divide between the Protestant Union and the Catholic League that had been formed in opposition. When Rudolf died the next year and Matthias became Holy Roman Emperor he started work on uniting the Habsburg lands by making Ferdinand the successor to his kingdoms.
When Ferdinand succeeded Matthias in 1619 he almost immediately went to war against Frederick V, the Elector Palatine and leader of the Protestant Union. Although there was virtually no pretext for this war Ferdinand had secured the support of Philip of Spain, for whom the Palatine would offer a direct road into the rebellious Netherlands, and the non-intervention of the Poles.
Although the members of the Protestant Union marshalled forces in support of the Palatine Frederick was quickly defeated by the Spanish Army of Flanders and Ferdinand's own not inconsiderable army. After subsequent defeats of the Army of Würtemberg and the Army of Brandenburg the power of the Protestant Union was broken and it's members defected in turn.
Although peace negotiations dragged on for several years Ferdinand was able to secure exceedingly favorable terms at the Treaty of Prague. He was crowned king of the Palatine abolishing the ancient title of Elector and his kingdoms were inexorably tied to the Title of Holy Roman Emperor. The defunct Protestant Union was also formally abolished.
While Ferdinand would not play any further role in the armed conflict his role was vital in the reconquest of the Netherlands. Allowing the Spanish to march reinforcements directly to the Dutch border and allowing simultaneous invasions from the South and the West.
Although Ferdinand would spent most of his effort on combating protestantism in the lands directly under his control, his real legacy was in strengthening the Holy Roman Empire. His successors would build on his work, steadily decreasing the number of German states and increasingly centralising control in the capital Prague. As the power of Spain waned the Empire increasingly became the dominant European power alongside France.
In reality, Rudolf II conceded the kingdom of Bohemia to his brother, the Bohemian revolt only happened in 1618 when Ferdinand stood to takeover the the kingdom. And from there it quickly spread across Europe and leading to the 30 Years War which devastated much of Europe. The position of the Holy Roman Emperor was fatally weakened and Germany broken up in countless independent states.