In 1655, King Charles X Gustav began a campaign against the Polish–Lithuanian Commonwealth to force King John II Casimir Vasa to renounce all claims to the Swedish crown, as well as conquer the Polish provinces of Courland and Prussia. The Swedish empire would be expanded, while the control of the lucrative Baltic Sea trade strengthened. However, the war against Poland was slow. Despite several tactical victories on the battlefield, including the conquest of Warsaw, Charles X Gustav was unable to bring the war to an end. The Poles resisted the Swedish troops, who were being ambushed constantly by Polish guerrilla units. Poland's neighbors threatened to get involved in the war. Both Austria and the Netherlands sent military aid to Poland, and Sweden's former ally, Brandenburg-Prussia, changed sides during the conflict. In 1656, Russian troops had crossed the border into Swedish Livonia and besieged Riga.