The last half of the 1500's saw Europe embroiled in a number of religious conflicts. For the most part, these wars were either between two countries (e.g., England vs. Spain, the Dutch vs. Spain) or internal affairs with some outside interference (e.g., France and Germany). However, as the seventeenth century dawned, religious and political tensions grew to encompass all of Europe in an interlocking network of states extending from Russia to England and from Sweden to Spain. These tensions exploded into what can be seen as the first European wide conflict in history: the Thirty Years War (1618-48).
The roots of the Thirty Years War extended back to two main developments in the 1500's: the religious wars emanating from the Protestant and Catholic Reformations, and fear of Hapsburg Spain and Austria, who between them controlled nearly half of Western Europe. Religious tensions (complicated by political rivalries) led to conflicts between Lutheran Sweden and Catholic Poland, German Protestants and Catholics, and the Protestant Dutch and English against Catholic Spain. Fear of the Hapsburgs also contributed to the English and Dutch conflicts with Spain. In addition, France, once it had recovered from its own religious wars, increasingly took the lead against the Spanish and Austrian Hapsburgs who ringed its borders to the north, south, and east. Venice also had problems with Austria over pirates in the Adriatic.
All these tangled religious and political tensions of the early 1600's polarized Europe into two camps defined largely, but not exclusively, by religion. The Protestant camp consisted of German Protestants, Denmark, the Dutch Republic, England, Sweden, Catholic Venice, and Catholic France. The Catholic camp had German Catholics, Spain, Austria, the Spanish Netherlands, Naples, Milan, the Papacy, and Poland.
Two such hostile camps staring menacingly at one another led to the common fear and expectation of a general war embroiling all of Europe. As a result, kings and princes built up armies and fortifications in preparation for the coming war, which merely reinforced the other side's fears of war, triggering more military spending and so on. Travelers of the time noted how states all over Europe seemed to be armed to the teeth and ready for a fight. This was especially true in Germany where the Protestant princes formed a defensive league known as the Protestant Union in 1609 while the Catholic princes quickly answered with the Catholic League.
Added to this were two other factors making Europe's economy less vibrant than it had been in the 1500's. For one thing, the flow of silver from the Americas had passed its peak. For another, the climate turned colder, reducing crop yields and straining Europe's ability to feed its population (which had doubled since 1450). This, in turn, led to lower resistance to disease (including Bubonic Plague which made a comeback in the 1600's). The combination of soaring military budgets, declining silver production, and the effects of a colder climate led to rising tensions in Europe, both between different states and between social classes within individual societies.
These problems combined with the fact that Europe was split between two hostile political/religious camps meant that any conflict or crisis between individual members of each camp could drag in all the other members of their respective camps and trigger a European wide war. In this respect, the situation largely resembled the one that would drag Europe into World War I in 1914.
In 1618, Protestants in Bohemia, then part of the Holy Roman Empire, rebelled against the Austrian Hapsburgs. Unfortunately, Germany's fragmented political situation generated a vicious cycle that would turn a local struggle into a European wide conflict using Germany as its battleground. As the crisis grew, more states would get involved and commit increasing amounts of resources. As more allies joined each side, the war grew into an exhausting stalemate that neither side could either win or afford to quit since it had already spent so much on it and felt it had to recover its expenses from its enemies. Concern over a Protestant or Hapsburg Catholic victory and belief that the balance could be tipped to their advantage would draw in more powers, eat up more resources, perpetuate the stalemate, and so on.
Thus Spain, Poland, the German Catholics, and the Pope came to Austria's aid to crush the Bohemian rebels. This caused Denmark, England and the Dutch Republic to join the conflict against the Hapsburgs and were defeated. Then Sweden attacked Austria, supposedly in defense of the German Protestants, but was eventually defeated. Finally, Catholic France threw itself into the fray, helping the Protestants against the Hapsburgs. Each new power that would get involved merely fed more fuel into the veritable firestorm of continuing stalemate until there was hardly anything left to burn.
More and more, this has become the pattern of modern warfare, as its expense makes it too expensive to fight, but also too costly to back out once a country has committed itself to it. And as the cost and destructiveness of war goes up, the spoils of war to make it pay for itself dwindle correspondingly. This dilemma has increasingly plagued modern warfare to the present day as the technology of war has gotten progressively more destructive and expensive, both to build and use.