The Protestants claimed the status of these lands fell under the term "royal land", and thus were subject to Bohemia's authority by the Letter of Majesty – a very disputed legal interpretation which the Habsburg government rejected. On 23 May 1618, an assembly of Protestant noblemen, led by Count Jindřich Matyáš Thurn, stormed Prague Castle, and seized two Imperial governors, Vilem Slavata of Chlum and Jaroslav Borzita of Martinice. The rebels charged them with violating the Letter of Majesty, found them guilty, and threw them and their scribe Philip Fabricius out of the windows of the Bohemian Chancellery. This event – known as the Second Defenestration of Prague – marked the beginning of the Bohemian Revolt, and with it, the beginning of the Thirty Years' War. In these circumstances Christian I, Prince of Anhalt-Bernburg, Frederick V's governor of the Upper Palatinate, moved to intervene in Bohemia. He did not initially propose nominating Frederick as king because the young elector was still seen as politically inexperienced, and was a Calvinist, while there were virtually no Calvinists in Bohemia. At any rate, Frederick was not initially eager to defy the Emperor, who had praised Frederick's loyalty. Frederick did not publicly break with the Emperor, but in a letter to his father-in-law, James I of England, he placed the blame for the Bohemian vote on the Jesuits and the Spanish party at the Habsburg court. This was a questionable evasion of the role played by Frederick's own agents. The first mention in Prague of Frederick's name as a possible candidate as king of Bohemia came in November 1618. It is not known if Frederick's agents played a role in talking up his possible candidacy. Palatine diplomat Christoph von Dohna approached James I of England with the possibility of Frederick becoming king, but James reacted negatively to this idea. The princes of the Protestant Union similarly rejected the idea, fearing it might lead to religious war and the Elector of Saxony was staunchly opposed. Behind the scenes, Frederick authorised sending a force under Count Ernst von Mansfeld to support the Bohemian rebels. In August 1618, forces under Mansfeld entered Bohemia and led the Siege of Pilsen, which saw Pilsen fall to rebel forces on 21 November 1618, leaving the entire kingdom in Protestant hands. Matthias, Holy Roman Emperor died on 20 March 1619.