False Dmitry II
Despite 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 Russian 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.