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Peace negotiations between France and the Habsburgs, provided by the Holy Roman Emperor and the Spanish King, were to be started in Cologne in 1636. These negotiations were blocked by France.
Cardinal Richelieu of France desired the inclusion of all its allies, whether sovereign or a state within the Holy Roman Empire. In Hamburg and Lübeck, Sweden and the Holy Roman Empire negotiated the Treaty of Hamburg. This was done with the intervention of Richelieu.
The Holy Roman Empire and Sweden declared the preparations of Cologne and the Treaty of Hamburg to be preliminaries of an overall peace agreement. This larger agreement was to be negotiated in Westphalia, in the neighbouring cities of Münster and Osnabrück. Both cities were to be maintained as neutral and demilitarized zones for the negotiations. Münster was, since its re-Catholization in 1535, a strictly mono-denominational community. It housed the Chapter of the Prince-Bishopric of Münster. Only Roman Catholic worship was permitted. No places of worship were provided for Calvinists and Lutherans.
Osnabrück was a bidenominational Lutheran and Catholic city, with two Lutheran and two Catholic churches for its mostly Lutheran burghers and exclusively Lutheran city council and the Catholic Chapter of the Prince-Bishopric of Osnabrück with pertaining other clergy and also other Catholic inhabitants. In the years of 1628-1633 Osnabrück had been subjugated by troops of the Catholic League. The Catholic Prince-Bishop Franz Wilhelm, Count of Wartenberg then imposed the Counter-Reformation onto the city with many Lutheran burgher families being exiled. While under Swedish occupation Osnabrücks's Catholics were not expelled, but the city severely suffered from Swedish war contributions. Therefore Osnabrück hoped for a great relief becoming neutralised and demilitarised.
Both cities strove for more autonomy, aspiring to become Free Imperial Cities, so they welcomed the neutrality imposed by the peace negotiations, and the prohibition of all political influence by the warring parties including their overlords, the prince-bishops.
Since Lutheran Sweden preferred Osnabrück as a conference venue, its peace negotiations with the Empire, including the allies of both sides, took place in Osnabrück. The Empire and its opponent France, including the allies of each, as well as the Republic of the Seven United Netherlands and its opponent Spain (and their respective allies) negotiated in Münster.
Read more about this topic: Peace Of Westphalia