Although held by a Habsburg since 1470, the Holy Roman Emperor was chosen by seven Prince-electors; Mainz, Cologne, Trier, Saxony, Brandenburg, Bohemia and the Palatinate.

Many German Protestants remained neutral, viewing it as an inheritance dispute and with Bavarian support, Emperor Ferdinand quickly suppressed the Bohemian Revolt.

In 1625, the Protestant Christian IV of Denmark invaded Northern Germany but after some success, was defeated by the Imperial general Wallenstein and forced to withdraw in 1629.

Ferdinand now enacted the Edict of Restitution; this required any property that had changed hands since 1552 restored to its original owner, which in nearly every case meant the Catholic Church.

[6] Richelieu relied on his ability to restrain the Swedish king, but Maximilian was less confident; Gustavus had quickly established his superiority over the Imperial armies, and insisted Frederick be restored.

To exclude France and Sweden from Germany, he suggested a 'Habsburg alliance', consisting of Spain, Austria, the Spanish Netherlands, and as many German states as possible, Catholic or Protestant.

[8] Clauses One and Two set the period of the agreement as eight years, with each party providing the other military assistance, if their 'inherited or acquired' territories were attacked, which essentially confirmed Bavarian possession of the Palatinate.

'[10] On 20 May, ten days before the treaty was signed, forces of the Catholic League, sacked the Protestant town of Magdeburg; over 20,000 were alleged to have died in the most serious atrocity of the entire war.