At a meeting of Protestant Princes in Lauenburg upon Elbe in 1625, it was decided to put Lower Saxony under the protection of King Christian IV of Denmark against the Emperor and his Catholic League.

Francis Charles recruited a new regiment for the Danish King and quartered it in his brother's neutral Saxe-Lauenburg, where his troops took a hostile stance.

After King Gustavus II Adolphus of Sweden had landed in Pomerania in 1630, Francis Charles entered his service as a colonel.

Francis Charles was captured at the residence of his brother Augustus in Ratzeburg by the imperial general Pappenheim, but was soon back in Swedish service as a colonel.

After the death of his first wife, and after imperial mediation Francis Charles married on 27 August 1639 in Sopron Catherine of Brandenburg, the very rich widow of the Transylvanian prince Gábor Bethlen.