Shuysky signed the Treaty of Vyborg with King Charles IX of Sweden on 28 February 1609, establishing a military alliance with the Polish–Lithuanian Commonwealth's main rival. Russia agreed to cede Korela Fortress and Kexholm County to Sweden in exchange for military assistance in fighting False Dmitry II and the Poles. Sweden launched the De la Gardie campaign, commanded by Jacob De la Gardie and Evert Horn, a 5,000-person force to assist the Russians under Mikhail Skopin-Shuisky. King Sigismund declared war on Russia, hoping to gain territorial concessions and weaken a Swedish ally, and Polish troops crossed the Russian border to besiege Smolensk. Although the Swedish-Russian alliance had several victories against Dmitry, it was defeated by the Poles at the Battle of Klushino on 4 July 1610 (ending the campaign).