at the age of 14, björnstjerna was sent to sweden, and attained the same year the rank of ensign ("swedish: fänrik") upon enrolling in the king's guard. during the finnish war he was the head adjutant at the finnish army's general staff, and participated in the major military campaigns during the war. the same year he was appointed major in the västmanland regiment. he was one of the most prominent participants in the swedish revolution of 1809; when it ended he was sent on a secret mission to napoleon i.