Charles Frederick of the line of the Dukes of Holstein-Gottorp married Grand Duchess Anna Petrovna of Russia, daughter of Peter the Great.

Another brother, Prince Georg Ludwig of Holstein-Gottorp, was the father of Peter I, who became Grand Duke of Oldenburg in 1823.

The genealogy of the counts of Oldenburg can be traced to the Saxon hero Widukind (opponent of Charlemagne)[citation needed], but their first historical representative was Huno of Rustringen (died 1088, founded the monastery of Rastede in 1059).

The Free Hanseatic City of Bremen and the bishop of Münster also frequently warred with the counts of Oldenburg.

Jever had been acquired before his ascension, but in 1624 he added Knipphausen and Varel to his lands; thus, in 1647 Delmenhorst was finally united.

Through neutrality during the Thirty Years' War and by donating valuable horses to warlord Count of Tilly, Anton Günther protected his dominions from the devastation levied on nearly all other German states.

Oldenburg was a wealthy town in a time of war and turmoil and its population and power grew considerably.

However, after the death of Anton Günther, Oldenburg fell again under Danish authority, and in 1667 the town was struck by a disastrous plague epidemic and shortly after that was destroyed in a fire.