After his father's death his first task as Elector of Brandenburg was to restore order, then get rid of the Swedes. The peace treaty of Westphalia in 1648 was a success for him; he lost some parts of 
Pommern but gained other territories. The problems between Sweden and Poland gave him the chance to obtain recognition for the sovereignty of his duchy of Prussia. Also, the continuing changes in politics combined with military action improved the standing of Brandenburg. 
Friedrich Wilhelm, "The Great Elector", changed the running of his territory and gained more control of the finances. The local nobility relinquished their political power but gained economic and social 
dominance of the countryside. Calvinists and Lutherans were treated equally; however, he was unable to make them adopt tolerance towards each other. 
His first wife, Princess Louise Henriette of Orange, accepted him with the utmost reluctance, having been forced by her mother to accept him instead of the penniless French cousin she loved. Nevertheless, she shared his work and bore him children but died aged only thirty-nine. 
Almost a year later he remarried Dorothea of Holstein-Glucksburg, a childless widow who bore him more children. However, in her endeavours to obtain properties for her sons, she was feared as "the wicked step-mother" by the children from the first marriage. When Ludwig, his second son died, after having been in her apartments, poison was feared and the eldest son and his wife fled from Brandenburg. However, Dorothea obtained only small properties and, on his death, Brandenburg was still given to his eldest son. 
 

Source: Leo van de Pas