The Peace of Westphalia, also known as the treaties of Münster and Osnabrück respectively, refers to the series of treaties that ended the Thirty Years' War, the Eighty Years' War, and "officially" recognized the United Provinces. The treaty was signed October 24 1648, and meant an end to the long conflict between Catholic and Protestant forces.
The peace negotiations were after initial talks held in the cities of Münster and Osnabrück, an alternative favoured by Sweden (Hamburg and Cologne being the French alternatives). The two locations were needed as the Protestant and Catholic leaders refused to meet each other. The Treaty of the Pyrenees ending the war between France and Spain is also often considered part of the treaty.
The results of the treaty were wide ranging. Among other things, the Netherlands gained independence from Spain, ending the Eighty Years' War, and Sweden gained Pomerania, Wismar and Bremen-Verden. The power of the Holy Roman Emperor was broken, and the rulers of the German states were again able to determine the religion of their lands. The treaty also gave Calvinists legal recognition. Three new great Powerss arose from this peace: Sweden, the United Netherlands and France. Sweden's time as a Great Power was to be short lived, however.
The majority of the treaty can be attributed to the work of Cardinal Mazarin who was de facto leader of France at the time. France came out of the war in a far better position than any other Power and was able to dictate much of the treaty.
It is often said that the Peace of Westphalia initiated the modern fashion of diplomacy as it marked the beginning of the modern system of nation-states. Subsequent wars were not about reasons of religion, but rather focused on reasons of state. This allowed Catholic and Protestant Powers to ally, leading to a number of major realignments.
Another important result of the treaty was it laid rest to the idea of the Holy Roman Empire having secular dominion over the entire Christian world. The nation-state would be the highest level of government, subservient to no others.
See also: History of Sweden 1648-1700