Succeeded his father Charles XI in 1697 and from 1700 was involved continuously in war with Denmark, Poland, and Russia. Charles XII was one of the most brilliant military strategists of all time. His tactics during the Great Northern War (1700-1721) were startling and effective.

He won a brilliant succession of victories, until in 1709, while invading Russia, he was defeated at Poltava, and forced to take refuge in Turkey until 1914. He was killed in 1718 while besieging Frederiksten.

The King had eclectic taste when it came to men. Among his better known lovers were Axel Wachtmeister, Prince Maximillian of Wurttemberg, General Rehnskold, and General Stenbock.