In January 1758, despite his victory over the Austrians, Frederick the Great found himself threatened once again. Despite their impressive stand at Gross-Jagersdorf in July 1757, Frederick did not take the Russian threat seriously. Fermor's Russians were laying siege to the fortress of Kustrin. Frederick reached the Oder and marched north, placing his army across Fermor's line of communication and supply. Fermor abandoned the siege and marched to Zorndorf. On 25 August, Frederick attacked. The Russian infantry turned to meet this threat and refused to buckle until General Seydlitz threw his cavalry against the Russian right flank. The Russians broke and fled. Frederick had managed to temporarily stave off the Russian threat but his illusions about Russian competence were shattered.