Charles X was king of Sweden from 1654 to 1660. Charles X was the man appointed by Christina to succeed her despite the objections of the nobles lead by Oxenstierna. Charles was the oldest son of the count Palatine of Zweibruken and Catherine, sister of Gustavus Adolphus.
As king, he inherited a difficult situation. The traditional relationship between the nobles and the crown, as established by Gustavus, had been severely strained by Christina. The nobles had also made it clear that they were not supportive of Christina's choice when she was queen. Now they had to work with the man they had not supported.
To save the crown from bankruptcy, the nobles did agree to give back to the crown some of the crown land they had acquired very cheaply. This did a lot to boost the relationship between Charles and the nobility. The lower estates were also kept happy because of this as it was an example of the nobility doing something to give the country a boost which cost them something out of their pockets. Also the lower estates had suffered badly from high taxation - now, it seemed that the nobles would have to do their 'bit' for the crown.
Charles decided to be ruthless and decisive with the nobility. In 1655, he forced through a policy of resumption - the regaining of crown land. This was driven not by the nobility handing back what they felt was appropriate, but by the crown - taking back what Charles felt was rightly his. He was fully supported in this by the lower estates. A law was pushed through the Diet ordering the nobles to hand back 25% of the lands acquired by them since November 1632. This would have had serious consequences for the nobles but the commission appointed to carry out this law achieved little as war monopolised the government's time - the War of the North.
Charles X started the war by attacking Poland lead by John II Casimir. No-one is actually sure why Charles ordered this attack. Three theories have been put forward.
One theory is that he did consider Russia the only real threat to Sweden in eastern Europe. He feared that Russia might attack and occupy Poland thus presenting a much bigger threat to Sweden. By defeating and occupying Poland, Sweden could stop a westward expansion by the Russians.
Another reason could have been that John II Casimir was a Catholic and consistently claimed to be the rightful heir of the throne in Sweden. By defeating Poland, this claim would also be defeated.
The final theory is that the people of Sweden were in turmoil and a successful war was the best way of united the people around the crown.
Charles X launched a two-pronged attacked on Poland. In a series of swift attacks, he took west Poland and moved on to the Vistula valley. A second attack took the Swedish army deep into Lithuania. In August/September 1655 he took Warsaw; Cracow fell in October 1655. In January 1656, Charles signed an alliance with Frederick William, the Great Elector of Brandenburg. Faced with such an opposition, John II Casimir fled.
Sweden, once again, appeared to be dominant in eastern Europe and the Baltic. Realising this, Alexis of Russia attacked Sweden in 1656 but this was not successful.
John II Casimir had no disappeared though. He played on the heresy of Sweden and created a catholic alliance to defeat the "heretic". In 1656, John had some success against the Swedes in Poland.
The whole scenario changed in June 1657 when Denmark, encouraged by Austria and Holland, declared war on Sweden. The reason was clear - Denmark wanted to be supreme in the Baltic. Frederick William broke his alliance with Charles and after signing the Treaty of Wehlau, joined Poland in September 1657.
Charles took the fight to Denmark. He attacked, and aided by an appalling winter which froze the sea, he moved from island to island capturing undefended towns and cities. Copenhagen and Zeeland were both threatened. Fearing much damage to their undefended capital, the Danes sued for peace. The Treaty of Roskilde of 1658, gave Sweden Skana, Halland, the Isle of Bornholm and returned to Sweden Swedish territories of Trondheim and Bohuslan. Sweden now had complete control of the area north of the Danish Sound.
Charles wanted more out of Denmark but they were unwilling to give away more than they already had. Loosing patience with Denmark, Charles launched a full-scale invasion in August 1658. The Dutch and Brandenburg helped the Danes and the capital Copenhagen held out against attack.
To launch one final push Charles needed money. He summoned the Diet to Goteborg where he was based. Here he suddenly died. His son, the future Charles XI, was only 4 so a regency took over which ended all the wars Sweden was involved in.
In the Treaty of Oliva (1660), John II Casimir gave up his claim to the Swedish throne. Sweden handed back to Poland all the land captured since 1655. The Great Elector was recognised as the sovereign of East Prussia.
In the Treaty of Copenhagen (1660), Denmark recovered Trondheim and Bornholm and Sweden agreed to end her belief that the Sound should be closed to all foreign warships.
In the Treaty of Kardis (1661), there was an agreement that Russia had not gained nor should gain any foothold on the Baltic coast.