After defeating his cousin, he would form the Corpus Evangelicorum, which he led at the Alte Veste and to victory at Breitenfeld.  
In his first year of rule, he avoided war with one of his neighbors with the Peace of Knared.  Shortly thereafter he sent an abortive expedition to seize the fortification of Dunamunde in an attempt to overthrow his initial rival for the throne, Sigismund III.  After defeating his cousin, he would form the Corpus Evangelicorum, which he led at the Alte Veste and to victory at Breitenfeld.  FTP, name this king who became known as the "Lion of the North," until he was capped at Lutzen while leading his Swedish forces in the Thirty Years' War.
In his first year of rule, he avoided war with one of his neighbors with the Peace of Knared.  
Gustavus Adolphus or Gustav II Adolph