The Empire had settled on the principle of rulers determining their principalities' religion, but then Habsburg king of Bohemia and 'soon to be elected' Emperor Ferdinand's attempt to impose his Catholicism on the mainly protestant Bohemians (against the oath of his enthronement) caused an escalation of violence spreading through Germany beyond his financial means: the Thirty Years War wrecked the country and killed a third of the population. The Bohemian nobleman Albrecht von Wallerstein offered an alternative: an army of 50,000 mercenaries paid for by being allowed to plunder and exacting all of their needs from the occupied (protestant) territories. He repelled the Danish invasion, but was deposed after his fellow Catholic electors joined his opposition and awarded the Mecklemburg duchy. Swedish King Gustaf Adolf Wasa (invading South to Bavaria) forced Wallenstein to be reinstated, yet his autonomous negotiations with the imperial protestants caused the Emperor to have him proscribed. After his murder, the Prague Peace Treaty proved pointless as neighbors France and Sweden wouldn't allow the Empire to be welded together. Only in 1648 an unprecedented pan-European congress in Münster and Osnabrück reached the Westphalian Peace.