Bernard received an unusually good education and studied briefly at the University of Jena, but soon went to the court of Duke John Casimir of Saxe-Coburg to engage in knightly exercises.

At the outbreak of the Thirty Years' War he took the field on the Protestant side, and served under Mansfeld at Wiesloch (1622), under the Margrave of Baden at Wimpfen (1622), and with his brother William at Stadtlohn (1623).

[1] When King Gustavus Adolphus of Sweden landed in Germany Bernard quickly joined him, and for a short time he was colonel of the Swedish life horse guard.

[2] At first as a subordinate to his brother William, who as a Swedish lieutenant-general succeeded to the command, but later as an independent commander, Bernard continued to push his forays over southern Germany; and with the Swedish General Gustav Horn he made in 1633 a successful invasion into Bavaria, which was defended by the imperialist general Count Aldringer.

Bernard had in the first instance received definite assurances from France that he should be given Alsace and Haguenau, Würzburg having been lost in the debacle of 1634; he now hoped to make Breisach the capital of his new duchy.