Brandenburg-Prussia
... that in some areas of Brandenburg-Prussia, only 10% of the population survived the Thirty Years' War?
... that in some areas of <b id="mwA-U"><a href="./Brandenburg-Prussia" id="mwA-Y" rel="mw:WikiLink" title="Brandenburg-Prussia">Brandenburg-Prussia</a></b>, only 10% of the population survived the <a href="./Thirty_Years'_War" id="mwA-c" rel="mw:WikiLink" title="Thirty Years' War">Thirty Years' War</a>?