Thirty Years War was a bloody religious war in Europe, It is biggest religious war in Europe in history and the most destructive war of the human history (HREball lost 6 million people from its poulation of 16 million people, thats almost a third of the HRE!).
Introduction
The European Wars of Religion had been progressively raging for two centuries, gradully growing in intensity. From the 
Hussite Wars of the early fifteenth century to the 
French Wars of religion in the late sixteenth century, it seemed as though a decisive conflagration was inevitable. The Thirty Years War concluded as one of the most devastating conflicts in the history of the world, but began as a local religious conflict. By its end, Europe, its religion and its borders had been forever changed, and upwards of eight million people had perished. What is the story of this cataclysmic conflict and how did it begin? We shall begin our series on the Thirty Years War with the Bohemian Revolt , and the Battle of White Mountain .
Background
Hussite Wars and Reformation
The first Bohemian unrest against Catholic rule, the Hussite Revolt (Hussite Wars), concluded in 1434 at the Battle of Lipany, delaying the reformation for a century. In 1516, a Dominican Friar came to the came to the Holy Roman Empire city of 
Wittenbergball in order to sell 'indulgences' - material ways to reduce the amount of punishment one had to undergo due to their sins. In the late-middle ages, this practice had become corrupt and commercialized. A german theologian, monk and priest, Martin Luther, objected, and famously nailed his 'ninety-five theses' to the Wittenberg church. Perhaps his most revolutionary work was his translation of the traditionally Hebrew and Greek bible into common German, allowing laymen to read it for themselves, rather than relying on their priests. This began the Protestant Reformation.
Early Religious Conflicts & And Tensions
The sixteenth century conflicts precipitated by this reformation began with the massive German Peasants' War of 1524 to 1525 - a religious conflict exacerbated by political and economic factors, a precursor of what was to come. This period of violence, which also included the Munster rebellion of 1532 to 1535 and the Schmalkadic Wars of 1546 to 1555 seemingly came to an end with the Peace of Augsburg - during which Holy Roman Emperor Ferdinand the First established the Key principle of 'Cuius regio, eius religio'. This allowed the many princes of the various provinces of the Empire to impose their own religion, either Catholicism or Lutheranism, on their own territory. It stabilised the situation at first, but there were problems. The 1580s saw the situation deteriorate even further under the rule of Rudolf II. The causes of this are complex, but it was probably a combination of severe climatic stresses on the European economy, external conflicts, such as the ongoing Eighty Years War, and an intensification of religious extremism. In 1606 another key moment came when the Treaty of Zsitvatorok was signed between the Habsburg Holy Roman Empire monarchy and the Ottoman Empire. This marked a decrease in pressure against the Empire's Balkan border due to the necessity for Ottoman soldiers to quell internal rebellion and to fight against the Safavids. This seemed like a victory for Christendom, but it also took away a unity factor between the various religious groups in the Habsburg's German territories, giving them room to focus on their internal rivals.
Tensions Rise
In 1608, Protestant princes declared the Protestant Union, a military alliance to defend the religious interests of their co-religionists under Frederick IV of the Palatinate. Feeling threatened by this, Catholic princes under Duke of Bavaria Maximilian the First formed the Catholic League. One of the largest provinces in the Empire, Bohemia, had enjoyed comparative religious freedom since the end of the Hussite Wars, and was one of the most tolerant lands in Europe. In 1609, Rudolf the Second issued the Letter of Majesty, a text which guaranteed freedom of conscience for all and liberty of worship for nobles, essentially allowing Bohemia the right to control its religious affairs free from Imperial interference, but it was clear that this was a fragile compromise. Rudolf the Second died in 1612 and was replaced by Matthias the Second, who was more radical in his religious policies, and began to slowly roll back to the right. The new Emperor also had no children, so the succession was an open question. As a four-year dispute over the guardianship of the Electoral Palatinate concluded in 1614, Frederick the Fifth became the Elector. He was a talented military leader and a zealous Calvinist, who firmly believed a Catholic conspiracy to annihilate German Protestantism, and positioned himself as leader of the Protestant princes. By 1617, Matthias fell ill and Ferdinand of Styria had been elected as the heir-designate, to the dread of the Bohemians. They had heard tales of Ferdinand's vicious counter-reformation as Archduke of Austria, and feared he would not allow non-Catholics in Bohemia. On June 6th, 1617, he ascended to the Bohemian throne as Ferdinand the Second, to the fury of Protestant Bohemians.
The First Decade of The War
Bohemian Revolt
Beginnings of the Revolt
What happened next is debated, but the Bohemians either wanted to perform a pre-emptive strike in anticipation of Ferdinand's predicted persecution, or violence was already done against the protestants. The issue of Habsburg rule over Bohemia was also a key factor. On the 23rd of May, 1618 an assembly of protestants went to the Hradshin - the royal castle - and insisted on seeing the Imperial governors. After a heated exchange of words between the assembly, led by Count Thurn, and the imperial representatives, the Imperials were thrown out of the council chamber's windows and fell - becoming infamously known as the Second Defenstration of Prague (which was commonly said outside of Bohemia, but more known as the Third Defenstration of Prague in Bohemia). This was the trigger for the Bohemian Revolt, and Europe would consequently be enrolled in an apocalyptic conflict for the next thirty years. This scandalous act in Bohemia and the brewing insurrection took the courts in Europe by surprise, including the Bohemians themselves. Despite the state of affairs, they were cautious, and refrained from demanding the deposition of Ferdinand for now, instead addressing their demands to the sickly Holy Roman Emperor Matthias, maintaining a pretense of loyalty. Nevertheless, military preparations and actions began immediately by both sides. In mid-June, the Bohemians wrote to the Protestant Union, asking them for full admittance and military support for the upcoming conflict. In order to incentivize this further, they offered Frederick the Fifth, the leader of the Union, the Kingship of Bohemia for their support. In a diplomatic mistake, they offered the same to the rulers of Savoy, Saxony, and Transylvania, which led the wider Union to declare neutrality in the conflict, contradictory to their leader - the Elector of the Palatinate. The ruler of Saxony - the most powerful Protestant province of the Empire - had never wanted a war with the Emperor, and when Ferdinand offered him any protection against any Protestant retaliation in return for neutrality, he accepted and mobilized his forces to secure his border with Bohemia. The Duke of Savoy however, responded to the Bohemian call for help swiftly, and sent a two-thousand strong force under Ernst von Mansfeld, along with financial aid for the conflict. In May 1618 this Protestant force captured the stronghold of Pilsen, marking one of the first major engagements of the Thirty Years War.