On this day...... 20 August

1336: The long-simmering rivalry between Philip VI of France and Edward III finally boiled over with the French King's declaration of his intent to invade England to aid his Scots allies, effectively marking the start of the Hundred Years War.

1765: John Harrison demonstrated his fourth chronometer to the Board of Longitude, set up at the Admiralty's behest to seek an accurate timepiece which could permit longitude to be measured accurately for navigation.  An award of £20,000 was offered.  Harrison's fourth chronometer design finally provided the breakthrough needed for precision timekeeping.

1914:  German naval codebooks were recovered by the Imperial Russian Navy from the wreck of the light cruiser Magdeburg, and passed to the Royal Navy - one of the most important intelligence coups of the First World War.  The Royal Navy's cryptographers in Room 40 were able to read German naval wireless signals for a considerable period of time thereafter, Royal Navy ships often being able to put to sea before German raiders had even left harbour.

1917: Although badly burnt by an enemy flamethrower, Second Lieutenant Parsons of the Gloucestershire Regiment mounted a single-handed defence with grenades against a German night attack long enough for reinforcements to arrive and hold the position.  He died of his appalling injuries and was awarded a posthumous Victoria Cross. 

Near Ypres, Second Lieutenant Moore, Hampshire Regiment led an assault on a formidable German position containing machine-guns and an artillery piece.  At the start, the British attackers numbered seventy, but by the time Moore took the position and some thirty prisoners, only himself and five others remained.  Nevertheless, they then held their ground for 36 hours.  Another fifty to sixty men reinforced them, but the relentless German attacks reduced them back down to ten, and they were eventually forced to withdraw.  Moore received the Victoria Cross.

1940: Luftwaffe activity remained at a low level, with the exception of one large raid in the afternoon.  The day's events are recorded on the RAF's Battle of Britain website.