While developing commercial significance of the Baltic Sea, the larger Northeastern European coun- tries began to fight for the convenient access to the Baltic Sea by occupying the most important sea - ports, the mouths of the rivers flowing into the Baltic Sea. The two principal and long lasting wars occurred between Sweden and Polish-Lithuanian state in the 17th century (1600-1629 and 1655-1660). During the period being analysed in this study, the king of Sweden Gustav the 2nd Adolph, ex- ploiting the difficulties that the Republic had in the Southeastern Europe (the war with Turks at Hotimo in 1621), brought himself to realize his intentions to occupy Livonia and dictate the king of the Republic an agreement according to which he would abdicate the Swedish throne. In this cam- paign of the Livonia War the king of Sweden Gustav the 2nd Adolph attempted to occupy Riga. The attack was successful because of the effec- tive action of combined artillery and infantry, as well as, the good knowledge of the city fortifica- tion. The surrender of Riga was the greatest loss of the GDL (Grand Duchy of Lithuania) during this campaign as well as the period of 1600 - 1629. The actions of the Swedish army had always been systematic. Fighting was based on siege at- tempting to occupy castles and fortify themselves. Besides, the possession of a powerful navy allowed the Swedish army to land at any place of the Duchy of Courland and Žiemgala. The Republic had no powerful navy to interfere the arrival of Swedish support. During the open battles the GDL cavalry was superior because the Swedish infantry of the army being reformed could not open sufficient fire which could queltch the GDL cavalry attack.