Claim: Swedish Empire was ruled by Gustav II Adolf for 21 years.

Gustav II Adolf -LRB- 9 December 1594 -- 6 November 1632 , O.S. -RRB- ; widely known in English by his Latinised name Gustavus Adolphus or as Gustav II Adolph , was the King of Sweden from 1611 to 1632 and is credited as the founder of Sweden as a Great Power -LRB- Stormaktstiden -RRB- . He led Sweden to military supremacy during the Thirty Years ' War , helping to determine the political as well as the religious balance of power in Europe . He was formally and posthumously given the name Gustavus Adolphus the Great -LRB- Gustav Adolf den store , Gustavus Adolphus Magnus -RRB- by the Riksdag of the Estates in 1634 .   He is often regarded as one of the greatest military commanders of all time , with innovative use of combined arms . His most notable military victory was the Battle of Breitenfeld . With a superb military machine with good weapons , excellent training , and effective field artillery , backed by an efficient government which could provide necessary funds , Gustavus Adolphus was poised to make himself a major European leader , but he was killed a year later at the Battle of Lützen in 1632 . He was ably assisted in his efforts by Count Axel Oxenstierna , the Lord High Chancellor of Sweden , who also acted as regent after his death .   In an era characterized by almost endless warfare , he led his armies as king from 1611 -LRB- at age 16 -RRB- until his death in battle in 1632 while leading a charge . Sweden rose from the status of a mere regional power to one of the great powers of Europe and a model of early modern era government . Within only a few years of his accession , Sweden had become the largest nation in Europe after Russia , Spain and the Polish-Lithuanian Commonwealth . Some have called him the `` father of modern warfare '' , or the first great modern general . Under his tutelage , Sweden and the Protestant cause developed a number of excellent commanders , such as Lennart Torstensson , who would go on to defeat Sweden 's enemies and expand the boundaries and the power of the empire long after Gustavus Adolphus ' death in battle .   He was known by the epithets `` The Golden King '' and `` The Lion of the North '' by neighboring sovereigns . He made Sweden one of the great powers of Europe in part by reforming the administrative structure . For example , he began Parish registration of the population , so that the central government could more efficiently tax and conscript the people . Historian Christer Jorgensen argues that his achievement in the field of economic reform , trade , modernization , and the creation of the modern bureaucratic autocracy was as great as his exploits on the battlefields . His domestic reforms , which transformed a backward , almost medieval economy and society , were in fact not only the foundations for his victories in Germany , but also absolutely crucial for the creation and survival of the Swedish Empire .