Operation Gambit

During World War II, Operation Gambit was a part of Operation Neptune, the landing phase of the invasion of northern France (Overlord). Gambit involved two X class submarines (British midget submarines) which marked the extreme left and right limits of the British and Canadian invasion beaches with navigation lights and flags off 'Sword' and 'Juno' beaches.

HMS X20 and HMS X23 arrived in position on 4 June and, due to the delay caused by bad weather, remained in position until 4:30am on 6 June (D-Day) when they surfaced, erected the navigational aids, an 18-foot telescopic mast with a light shining to seaward, a radio beacon and echo sounder tapping out a message for the minelayers approaching 'Sword' and 'Juno' beaches.

A similar operation had been offered to the US landing forces to mark their beaches, but this was declined.

The submarines were at some risk of damage due to friendly fire and in an attempt to counter this Lieutenant Honour the captain of HMS X23 acquired and flew an ensign of the size more normally used by capital ships.