He was a son of Charles XI of Sweden and Ulrika Eleonora of Denmark. He was brought up in the spirit of absolutism. Soon after his father's death on April 5, 1697 he denied regency and crowned himself as absolute monarch. Young Swedish King's military and political inexperience encouraged Russia, Denmark and Poland-Saxony in their hopes of regaining their lost possessions in the Baltic. An alliance between these countries was formed and Russia declared war on Sweden in 1700. This war called later as Great Northern war quickly revealed Charles's XII ability as a gifted military commander. Very soon he succeeded in forcing Denmark to make peace and leave alliance (1700). Then he defeated Russian army in the battle of Narva (1700), conquered Courland (1701) and invaded Poland. Later Charles XII overthrew Augustus II of Poland and Saxony and secured the election of Stanislaus Lezshinsky as a King of Poland. Since 1707 Swedish King has been concentrated on his main enemy Russian Tzar Peter I. He secured the alliance of the Cossack leader Hetman Mazepa and invaded Russia in 1708. By the mean time Russian army has been reorganized and well equipped with armament.

His plans to take Moscow were not realized because of the "scorched earth" tactics implied by Russians, the lack of food, gunpowder and of course the extremely severe winter of 1708-1709. Swedish detachment headed by General Lewenhaupt, that was sent from Riga to the main Swedish army with provisions and ammunition was defeated by Russians near the village of Lesnaya. Under such circumstances Charles XII took a decision to turn south into the Ukraine. His army reached Poltava in the April, 1709. On June 27, 1709, a decisive battle of the Great Northern war took place near the town of Poltava. Being defeated by superior army of Peter I and suffering heavy losses Swedish army retreated southward to the Dnieper River. Near the village of Perevolochna Russians настигли Swedes and forced them to surrender. About 16000 Swedish warriors were taken prisoner and then sent to Siberia. The King and a small detachment fled into the Ottoman Empire. Soon his status of honorary guest was turned to the status of prisoner. Being far from Sweden he administered the affairs of the state, changed taxation policy to accumulate more money for the military purposes. Such internal policy resulted in the impoverishment of Sweden and people's discontent increase. Charles XII was allowed to leave Ottoman Empire in late 1714, going first to Stralsund and returning to Sweden in 1716. By this Russia has extended un-Swedish coalition by joining Brandenburg and Hannover. Even after getting lost his Baltic provinces and possessions in Germany Swedish King still refuse to sign peace treaty with Russia. As result, the center of military operations was moved to Baltic. In many sea battles Russian fleet prevailed over Swedish one. In 1716 Charles XII made unsuccessful attempt to attack Norway. A 1716 campaign against Norway failed. A new campaign began in 1718, when Fredrikshald was laid in siege. On November, 30 1718, while inspecting the siege trenches, Charles was shot in the head and died instantly. Who killed him has remained a question for historians ever since. His body was carried to Stockholm and after embalming buried in the Riddarholm church on February 26, 1719. Charles XII has left no successors so he was succeeded by his sister, Ulrika Leonora.