fleet of the Dutch West India Company, under Piet Hein, which captured the Spanish treasure convoy off the Cuban coast, at Matanzas, in 1628, further depressed Spain's fortunes, while it encouraged the Dutch. The war over the Mantuan Succession also acted as a massive diversion and took up much of the Spanish recources which would otherwise have been committed to the Netherlands. In 1629, 's-Hertogenbosch, one of the best fortified cities in Europe, was captured by the Dutch States Army, under Frederick Henry, Prince of Orange, despite Spanish and Imperial attempts to save the city. This was a shattering blow to Spanish prestige, which caused deep dismay in Madrid and represented the first large scale Spanish defeat in Europe since the scattering of the Armada of 1588. Swedish intervention; 1630 to 1634 Gustavus spent the period from 1626 to 1629 fighting against Poland–Lithuania, which was ruled by his Catholic cousin Sigismund, who also claimed the Swedish throne and had Imperial support. With only a few minor states like Hesse-Kassel still openly opposing the Emperor, Gustavus became an obvious ally for Richelieu. In September 1629, the latter helped negotiate the Truce of Altmark between Sweden and Poland, freeing Gustavus 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. Following failed negotiations with the Emperor, Gustavus landed in Pomerania in June 1630 with nearly 18,000 Swedish troops. Using Stralsund as a bridgehead, he marched south along the Oder towards Stettin and coerced Bogislaw XIV, Duke of Pomerania, into agreeing an alliance which secured his interests in Pomerania against his rival Sigismund. As a result, the Poles turned their attention to Russia, initiating the 1632 to 1634 Smolensk War. However, Swedish expectations of widespread German support proved unrealistic. By the end of 1630, their only new ally was the Administrator of Magdeburg, Christian William whose capital was under siege by Tilly. Despite the devastation inflicted by Imperial soldiers, Saxony and Brandenburg had their own ambitions in Pomerania, which clashed with those of Gustavus; previous experience also showed inviting external powers into the Empire was easier than getting them to leave. Gustavus put pressure on Brandenburg by sacking Küstrin and Frankfurt an der Oder, while the Sack of Magdeburg in May provided a powerful warning of the potential consequences of Imperial