1618, Sahaidachny joined the anti-Turkish Holy League. While he was battling the Turks, the Polish–Lithuanian Commonwealth requested his assistance for war with Muscovy (Russian Tsardom); they wanted him to provide Władysław IV Vasa, the King of the Polish–Lithuanian Commonwealth, with 20,000 Cossacks near Moscow. Sahaidachny did, and seized the forts in the cities of Putivl, Kursk, Livny, Yelets and many others. Near Serpukhov Sahaidachny forced the Muscovite army to flee. The Muscovite troops under command of the voivode G. Volkonsky forced the Cossacks to take a detour, but were unable to stop the advance of the Cossacks regiments to Moscow. In September 1618 he forced to flee the army of another Muscovite nobleman, Vasilii Buturlin. Later, the united army of Jan Karol Chodkiewicz and Sahaidachny sieged Moscow and on 11 October unsuccessfully attempted to take the Arbat Gates. In late October, the army of Sahaidachny moved in a raid towards the south from Moscow. During this raid the army captured the city of Serpukhov, and in early December it captured Kaluga. John III Sobieski wrote that this successful raid caused panic among the Russians and forced them to conclude negotiations as soon as possible. The whole campaign finally culminated in December 1618 by signing the Truce of Deulino, resulting in the greatest territorial expansion of the Commonwealth. The Cossacks' invasion of Muscovy has been described by Ukrainian historian Valery Smolyi as "not the best page of Sahaidachny's biography". Sahaidachy and his cossacks had been positioning themselves as supporters of the Orthodox Christianity and potential allies of Muscovy.<ref name="ursr.org">Valery Smolyi, "Peter Sahadachny: knight, politician, man". Vitchizna', 13, #1, 189-194 (1990) (Валерій Смолий, Петро Сагайдачний: воїн, політік, людина. Вїтчизна, 13, №1, 189-194 (1990))] </ref> However, they left the "bloody trace" which extended from Livny to Moscow and back to Kaluga and Kiev. In research of Russian, Ukrainian and American historians Cossacks have been blamed for destroying and robbing Orthodox churches, cities and villages, killing children and women, who belonged to Orthodox (Greek) Christianity. Later, Sahaidachny asked Patriarch Teophanes III of Jerusalem to forgive him for such behavior. Sahaidachny returned to Zaporizhia, and did not only become a kosh otaman, but was also the Hetman of Ukraine. (Another source claims that in 1621 he was a colonel of the Commonwealth Registered Cossacks regiment.) In order to avoid conflict with the Poles, Sahaidachny agreed to limit the Cossack register to 3,000 men; the remainder