The main challenge for Gustav II Adolf was to defend the Lutheran faith against the Catholics of the south (according to Swedish geography; "the south" was actually present-day Germany and Poland and should not be confused with today's southern Europe). To do this he realized that he had to defeat the Holy Roman Empire on the field of battle; this was the dominating Catholic state in Europe which had begun enforcing its faith upon small Protestant nations in Germany, most notably Sweden's ally of Pomerania. This action provoked Sweden to initiate an invasion, and after landing in allied territory the superior Swedish army easily defeated an Imperial force at Frankfurt one year after establishing a beachhead in northern Germany. Although the state of Magdeburg, one of Sweden's few allies in the region, were overwhelmed by an Imperial army and had their capital city burnt to the ground with its citizens slaughtered, this only served to make the Holy Roman army underestimate their adversaries, being crushed in the following battle of Werben by a less numerous Swedish force. But no truly decisive battle was fought until 17 September 1631, when the Swedish Royal Army supported by Protestant Saxony engaged an imperial army of 35,000 men at Breitenfeld, resulting in the destruction of roughly 70% of Tilly' army with fairly low casualties on the Swedish side.