Complex sentence: Subsequently, the Russian forces, commanded by Pyotr Saltykov, occupied Frankfurt an der Oder, Prussia's second largest city, on 31 July, and began entrenchment of their camp to the east, near Kunersdorf.
Split into simpler sentences: Subsequently, the Russian forces, commanded by Pyotr Saltykov, advanced the to occupy Frankfurt an der Oder, Prussia's second largest city, on 31 July. There he ordered entrenchment of their camp to the east, near Kunersdorf.