Godunov died in June 1605 and was replaced by False Dmitry I, whose popularity among the Russian populace declined rapidly during his reign, and the Polish withdrew when he was eventually murdered during an uprising in Moscow in May 1606. Despite of this, Russia's instability continued to the near-total breakdown of order, prompting the Polish to invade again in 1607 in support of the new usurper, False Dmitry II. In 1609, the Tsar of Russia at the time, Vasili IV, approached King Charles IX of Sweden to form a military alliance against False Dmitry II and the Polish occupiers. The two signed the Treaty of Viborg ("Vyborg"; Finnish: "Viipuri"), in which Russia ceded Kexholm County and the strategic Korela Fortress to Sweden in exchange for military support. This Russia's alliance formed in 1609 with Sweden, the main rival of Poland, led to King Sigismund III of Poland officially declaring war on Russia in response. In 1608–1613, De la Gardie as Sweden's Chief Commander in Finland also commanded the Swedish war efforts in Russia. Thus, in accordance with the Swedish-Russian military alliance formed in 1609, he together with Evert Horn now took charge of providing an auxiliary corps to support the Russian forces commanded by Mikhail Skopin-Shuisky. Although officially the Swedish-Russian alliance was not ratified before July 1609, already in the early spring of 1609 Sweden gathered for this mission to the city of Vyborg (Finnish: Viipuri) in Finland (then part of Sweden) c. 5,000 soldiers, consisting mainly of Finns. A Swedish offensive heading towards Moscow – via Novgorod – began from Vyborg on 11 March 1609. The operation became known as De la Gardie campaign. It was a joint military campaign by the Tsardom of Russia and Sweden during the Polish-Russian War (1605-1618), a.k.a. Polish–Muscovite War or the "Dimitriads", lasting officially from April 1609 to 4 June 1610. A combined Russo-Swedish army of about 10,000 soldiers set out from Novgorod in April 1609 and marched towards Moscow, defeating rebel forces and relieving the Siege of Troitse-Sergiyeva Lavra on their way. The De la Gardie campaign was successful against False Dmitry II, dispersing his court in Tushino, a former village and town to the north of Moscow, where Dmitry II maintained an alternative court, challenging the authority of Vasili IV.