After returning to the Lithuanian Commonwealth, they both voluntered in the army after the death of king Władysław IV Vasa to fight in the ensuing Cossack-Polish War. After the Swedish invasion of Poland Sobieski first joined the side of most regiments swearing alligiance to king Charles X Gustav of Sweden. He howevver defected again in March 1656 and joined king John II Casimir Vasa again. He became an important commander and joined the pro-French faction of Queen Marie Louise Gonzaga. In 1665 he married his French wife. His greatest popularity rested on his militairy successes against the Cossacks and his skilfull maneuverings during the unruely Polish diets (Sejm). Eventually he was elected kingof the Polish Luthuanian Commonwealth on 19 May 1674. Meanwhile his love for France cooled dramatically, resulting in a breach, and his gravitating more to Habsburg-Vienna in their struggles with the Ottoman Empire