On 6 June 1654, Charles X Gustav became the king of Sweden. This was a source of concern to Frederick, who considered that the new Swedish king's temperament would lead to much aggression. An opportunity came when in July 1655, the Swedish king started a campaign against Poland, the Polish War. This was a source of relief to Frederick, who was concerned about a direct attack on Denmark. He saw this as a distinct opportunity, and with his power in the Riksråd secured, he was able to convince them on 23 February 1657 to grant significant subsidies for a mass military mobilisation for a strike on Sweden while their army was overseas in Poland. On 23 April 1657, he got permission from the council to attack Swedish holdings in Germany, and war was formally declared on 5 June 1657. However, this proved to be a mis-step for the Danish king - the Swedish army quickly abandoned their polish campaign and invaded jutland, and then marched quickly across the frozen sea to unexpectedly conquer the rest of Denmark in the March Across the Belts.