In the 1930s German electronics manufacturers became involved in the then revolutionary idea of using radio beams for guiding aircraft in all weathers.
The principle was quickly adapted for medium-frequency use in American civil aviation.
A network of inter-city aerial routes was set up and expanded coast to coast. The system spread widely, and both pilots and radio operators became used to it. There were range and other limitations.
In June 1940 Churchill was told the Germans had a working system called Knickebein (crooked leg), which would guide bombers to a target in darkness or bad visibility.
Indeed a second, more complex and more accurate system, often referred to as X or X Gerat or X Verfahrung was in production.
The British started late but pumped in resources, and progress was rapid. Kenneth Wakefield's excellent The First Pathfinders shows that the "pathfinder" technique was first developed by the Germans but that the British and Americans had improved it out of all recognition before the end of the war.
Their OBOE beam navigation system was very good. The pathfinder technique used fast planes with highly-skilled navigators to go ahead of the main bombing force and illuminate the targets with incendiary bombs and burning buildings.
In 1941 the main German pathfinder unit was Kampfgruppe 100 (Battlegroup 100).
The Luftwaffe bombing of the North Strand on May 31st/June 1st, 1941, has been much discussed. After the war, on September 31st, 1945, the Belfast Telegraph reported that a beam was used for all the Belfast blitzes, and that RAF techniques had improved so that it was possible to say early in the evening which target had been chosen for that night. Target areas could thus be warned.
"On the evening of May 31st, 1941, the beam was again reported on Belfast. Counter-measures just devised were instituted and bent the German radio beams so that they converged over the open sea.
Some crews, finding they had been given the wrong co-ordinates, carried on to other targets by ordinary navigational methods, while one or more mistook Dublin for Belfast."
It has been claimed, and denied, that the beam was deliberately bent to cover Dublin. One would think that was not an easy thing to do at the time, although the RAF had made good progress in interfering with the beams by jamming and static.
Technical opinion now is that bending was a possibility, but there was too much other development to be done.
Wakefield summarises the German reports on most of the attacks on Britain and Northern Ireland for 1940-41, finishing in June 1941. He does not mention any raid for May 31st to July 1st.
There is a theory that Dublin was bombed as a warning not to send fire brigade units north again, or not to abandon neutrality.
This seems unlikely.
The Germans had every interest in not bombing this neutral State. They did pay compensation later and showed every sign of regret about the incident.
Whatever the truth, the propaganda stakes are so high that the truth is unlikely ever to come to light.