During the Second World War, the Firth of Forth was a vitally strategic waterway, of tremendous importance to both the Royal Navy and commercial shipping alike. In order to protect this estuary a number of defensive measures were taken. These included the provision of three separate lines of coast batteries as well as the laying of a number of minefields within the Firth of Forth.

Mines were laid along the southern shores of the Firth of Forth, the main channel for shipping lying along the northern shore and to the north of the Isle of May. These mines were designed to prevent any enemy ships from attempting to enter the Forth, and would have been detonated electrically from Inchkeith when an enemy ship was spotted over the minefield. Such measures were taken at all the east coast ports and minefields were laid along the length of the North Sea coast in an attempt to protect the east coast shipping lanes.

It was, however, essential to ensure that the minefields were not a hazard to friendly shipping. Consequently, the Royal Navy set up a radar station on the coast just north of Dirleton. This station had no connection with the Ground Control of Interception station at Dirleton, and lay a little to the north of the RAF's GCI installation. Nor did it have any direct connection with Fidra coast battery which lay nearby. The Naval station, known officially as Fidra even though it was not built on the island, was a Type 273 radar which operated on the very short wavelength of 10 centimetres. This gave the station a tremendous degree of accuracy enabling it to guide ships between the minefields without danger.

Fidra radar station was, however, not well sited. Its location on the mainland just opposite the island meant that much of the radar display was filled with returns from Fidra island itself, greatly reducing the efficiency of the radar equipment. As a result of these problems, the station closed down within a few months of becoming operational.

It was not only the British that were laying mines in the Firth of Forth. Extensive mine-laying was carried out by both the German Air Force and Navy, from a combination of Stavanger-based bombers and U-boats sailing from German and Norwegian bases. These proved difficult targets to detect, submarines being almost invisible to radar at the start of the war and mine-laying aircraft generally flying below the coverage of the radar chain to drop their mines. A considerable number of ships were lost in the early months of the war to magnetic mines laid at the entrances to the east coast ports and the Forth was no exception. On 21 November 1939 the 10,000 ton cruiser HMS Belfast had her back broken by a magnetic mine, almost splitting her in two and necessitating her virtual reconstruction at Rosyth. The desperate threat of the magnetic mine in 1939 and 1940 was eventually beaten when one was rendered harmless by two bomb disposal experts and its method of operation discovered. However, the Germans continued to drop mines in the Forth area, although not all of these landed in the estuary. On 24 July 1940 two mines landed on Traprain Law. It was also a mine-laying aircraft which, having laid its mines, dropped its supplementary load of bombs on Haddington on 3 March 1941, and not an aircraft returning from an attack on Clydeside, as has been suggested in some circles. There was, in fact, no attack on Clydeside on that date, bombing raids on that area not beginning until the night of 13/14 March.

Another measure taken by the Royal Navy for the protection of the Firth of Forth was the installation of an indicator loop system. This comprised two sets of electric cables, laid on the bed of the estuary about a mile apart. Whenever any large metal object, such as a ship, passed over the cables, the ship's magnetic field would create an electric current in the cables, which would be detected by sensitive equipment. From the separate cables, the direction in which the ship was moving could be determined. If prior notification of the passage of a ship had not been received, it was possible that there was an enemy ship or submarine attempting to enter, and the defences could be alerted accordingly. In many cases, when Royal Navy ships were sent to investigate, it was found that the vessel was a fishing boat which had failed to notify the authorities of its movements. The indicator loop system in the Forth was controlled from the Isle of May, the southern loop running from there to Canty Bay, near Tantallon. Another loop ran north from the island to the Fife coast, thereby covering the whole of the Forth estuary.

It can be seen that there was an extensive network of defences protecting the Forth during the Second World War and, thanks to the combination of minefields, coast artillery, indicator loops as well as air defences, no German ships ever attempted to enter the Firth of Forth and bombard Rosyth. That alone justifies the effort involved in creating these effective defences.