By this point Frederick was increasingly concerned by the Russian advance from the east and marched to counter it. Just east of the Oder in Brandenburg-Neumark, at the Battle of Zorndorf (now Sarbinowo, Poland), a Prussian army of 35,000 men under Frederick on Aug. 25, 1758, fought a Russian army of 43,000 commanded by Count William Fermor. Both sides suffered heavy casualties – the Prussians 12,800, the Russians 18,000 – but the Russians withdrew, and Frederick claimed victory. In the undecided Battle of Tornow on 25 September, a Swedish army repulsed six assaults by a Prussian army but did not push on Berlin following the Battle of Fehrbellin.
How did Frederick respond to the Russian occupation of east Prussia?
marched to counter it.