John of Werth, who was posted here, not being strong enough to dispute the passage of the Isar, fell back towards the Bohemian frontier, hoping to meet the troops which the emperor had urged Wallenstein to send to his aid, but which never came.

Duke Bernhard crossed the Isar unopposed, and on the 12th came within sight of Passau.
So far Wallenstein had not moved; he had seemed to comply with the emperor's request to save Ratisbon, but had seemed only, and had not set a man in motion to reinforce John of Werth.

He refused, in fact, to fritter away his army.

Had he sent Gallas with 12,000 men to join John of Werth, and had their united forces been, as was probable, attacked and defeated by the Swedes, Wallenstein would have been too weak to save the empire.

Keeping his army strong he had the key of the position in his hands.
He had fixed upon Passau as the point beyond which Duke Bernhard should not be allowed to advance, and felt that should he attack that city he and his army were lost.