He then joined the French Navy and took part in the Battle of the Saintes where he was taken prisoner by his former commander Admiral Sir Peter Parker and was imprisoned in England.

[1] According to the custom of the time, he obtained a letter of appointment at the age of ten and took leave together with several other officers from the Fleet of the Army for training purposes to join foreign war service in 1776.

[3] Rosenstein served in the Royal Navy 1776–78, where he became a midshipman and took an active part in Sir Peter Parker's squadron in the West Indies and participated in the Battle of Ushant against the French.

[3] As a French naval officer, he participated in the American Revolutionary War and was in the Battle of the Saintes on 12 April 1782 captured by his former commander Admiral Parker and was taken to England,[3] where he nonetheless soon was freed against his word of honor that he would not fight against the English.

During the Battle of Svensksund on 24 August 1789 he defended the passage aboard the hemmema Oden, until the rest of the fleet managed to escape, when he, after a ten-hour incessant cannonade at last, had to surrender and was taken as a prisoner to Russia.