Richelieu's policy was to 'arrest the course of Spanish progress', and 'protect her neighbours from Spanish oppression'. With French resources tied up in Italy, he helped negotiate the September 1629 Truce of Altmark between Sweden and Poland, freeing Gustavus Adolphus to enter the war. Partly a genuine desire to support his Protestant co-religionists, like Christian he also wanted to maximise his share of the Baltic trade that provided much of Sweden's income. Using Stralsund as a bridgehead, in June 1630 nearly 18,000 Swedish troops landed in the Duchy of Pomerania. At the same time, 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. As a result, the Poles turned their attention to Russia and into the 1632 to 1634 Smolensk War.