While the negotiations between the emperor and Wallenstein were pending Franz Albert was employed by the latter in endeavouring to bring about a secret understanding with the court of Dresden.
When Gustavus was blockaded in Nuremberg by Wallenstein Franz Albert left the camp of the latter and presented himself in that of Gustavus as a convert to the Reformed Religion and anxious to serve as a volunteer under him.

No quarrel or disagreement had, so far as is known, taken place between him and Wallenstein, nor has any explanation ever been given for such an extraordinary change of sides, made, too, at a moment when it seemed that Gustavus was in a position almost desperate.

By his profession of religious zeal he managed to win the king's heart, but Oxenstiern, when he saw him, entertained a profound distrust of him, and even warned the king against putting confidence in this sudden convert.
Gustavus, however, naturally frank and open in disposition, could not believe that treachery was intended, and continued to treat him with kindness.

After the assault made by Gustavus upon Wallenstein's position Franz Albert quitted his camp, saying that he was desirous of raising some troops for his service in his father's territory.

He rejoined him, however, with only his personal followers, on the very day before the battle of Lutzen, and was received by Gustavus with great cordiality, although the absence of his retainers increased the general doubts as to his sincerity.
He was by the king's side when Gustavus received his first wound.