The Diet of Regensburg was a meeting of the prince-electors of the Holy Roman Empire (or Kurfürstentag) which occurred at Regensburg from July to November 1630. It resulted in a major loss of power for the Holy Roman Emperor Ferdinand II.
The Thirty Years' War had prevented any imperial diet or reichstag from occurring for thirty years - the last one before the war had been that in 1613. In 1623 the emperor had summoned and dominated a Fürstentag at Regensburg. The rest of the states' corporate representation had been done on occasional days at the Imperial Circle and at the Kurfürstentags of 1619, 1627, 1630, 1636 and 1640.
The Diet of 1630 was preceded by Ferdinand's Edict of Restitution and the Peace of Lübeck after Wallenstein's defeat of Denmark. These both put the emperor in a favourable position to successfully achieve his goals.
Anselm Casimir Wambold von Umstadt was one of those invited to the Diet, as Imperial Chancellor (Reichserzkanzler) and Bishop-Elector of Mainz. The meeting opened on 3 July 1630 by Ferdinand himself. The Catholic electors were present in person, whilst those of Saxony and Brandenburg were represented by ambassadors.
Ferdinand's chief aim was to have his son Ferdinand the Younger elected King of the Romans and to gain the Empire's military support against the Dutch Republic in the Thirty Years' War, as well as against France in the War of the Mantuan Succession. He also needed to deal with the imminent Swedish threat - on 6 July, shortly after the Diet opened, Gustavus Adolphus of Sweden landed in Pomerania
Maximilian I of Bavaria feared the rising power of the empire and the strength of Wallenstein's Imperial Army. The Catholic electors therefore opposed rather than supported Ferdinand, requiring that the imperial army be downsized, the war-taxes reduced and Wallenstein dismissed from command. Ferdinand largely had to concede these demands so as not to lose his political underpinning in the empire. Wallenstein was dismissed and Johann Tserclaes, Count of Tilly put in supreme command of imperial forces, which was shrunk despite the Swedish threat. The debate on Mantua forced Ferdinand into a peace treaty which France soon broke. The electors blocked Ferdinand the Younger's election as King of the Romans and in the name of their own freedom revoked the Edict of Restitution.
The Diet thus saw Ferdinand - previously at the height of his power - suffer his first defeat by the Imperial States, a defeat that was near-complete.