prospered due to exports of timber, iron, and most notably copper, coupled with the advantages of a growing navy and proximity to the Livonian ports across the narrow Gulf of Finland. Before the Livonian War, Sweden had sought expansion into Livonia, but the intervention of the Russian tsar temporarily stalled these efforts through the Russo-Swedish War of 1554–1557, which culminated in the 1557 Treaty of Novgorod. Through its absorption of the principalities of Novgorod (1478) and Pskov (1510), the Tsardom of Russia had become Livonia's eastern neighbour and grown stronger after annexing the khanates of Kazan (1552) and Astrakhan (1556). The conflict between Russia and the Western powers was exacerbated by Russia's isolation from sea trade. The new Ivangorod port built by Tsar Ivan on the eastern shore of the Narva River in 1550 was considered unsatisfactory on account of its shallow waters. Thereafter the tsar demanded that the Livonian Confederation pay about 6,000 marks to keep the Bishopric of Dorpat, based on the claim that every adult male had paid Pskov one mark when it had been an independent state. The Livonians eventually promised to pay this sum to Ivan by 1557, but were sent from Moscow when they failed to do so, ending negotiations. Ivan continued to point out that the existence of the Order required passive Russian support, and was quick to threaten use of military force if necessary. He aimed to establish a corridor between the Baltic and the new territories on the Caspian Sea, because if Russia were to engage in open conflict with major western powers, it would need imports of more sophisticated weaponry. The Polish King and Lithuanian Grand Duke Sigismund II Augustus was wary of Russian expansionist aspirations. Expansion of Russia into Livonia would have meant not only a stronger political rival but also loss of lucrative trade routes. Therefore, Sigismund supported his cousin Wilhelm von Brandenburg, archbishop of Riga, in his conflicts with Wilhelm von Fürstenberg, the Livonian Order's landmeister. Sigismund hoped that Livonia, just like the Duchy of Prussia under Duke Albert, would become a vassal state of Poland–Lithuania. With weak support in Livonia, von Brandenburg had to largely rely on external allies. Among his few Livonian supporters was landmarschall Jasper von Munster, with whom he planned an April 1556 attack on his opponents that would involve military aid from both Sigismund and Albert. However, Sigismund hesitated over participation in