 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.At an Imperial conference held in August, Catholic hardliners succeeded in blocking concessions on the Edict; the combination meant Saxony allied with Gustavus and their combined army won an overwhelming victory over the League at Breitenfeld in September.This dramatically changed the balance of power and by early 1632, Swedish forces occupied much of Southwest Germany and Bavaria, including Munich.