The king made no formal declaration of war against the Catholic powers. After the attack that had taken place on Stralsund, his ally, he felt that he had sufficient pretext to land without declaring war. Using Stralsund as a bridgehead, in June 1630 nearly 18,000 Swedish troops landed in the Duchy of Pomerania. Gustavus signed an alliance with Bogislaw XIV, Duke of Pomerania, securing his interests in Pomerania against the Catholic Polish–Lithuanian Commonwealth, another Baltic competitor linked to Ferdinand by family and religion. Expectations of widespread support proved unrealistic; by the end of 1630, the only new Swedish ally was Magdeburg, which was besieged by Tilly.