 Complete the following phrase:  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 todays 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 Swedens 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 Swedens few allies in the region, were overwhelmed by an Imperial army and had 