After the German attack on Poland there was a period of inactivity on the Western front, between September 1929 and April 1940, punctuated by a few unsuccessful attempts by the Entente, the name for the Allies, to draw Germany into an offensive in the West. This period is often referred to as the 'Phoney War'. The Germans referred to it as 'Sitzkrieg', the "sitting war". The period saw the completion of the occupation of Poland and the transfer of German armed forces from the east to the west in anticipation of a German offensive against France.
The German U-Boats were very active during the so-called 'Phoney War', sinking vast amounts of Allied shipping.
The 'Phoney war' only refers to the land war. At sea the Battle of the Atlantic was starting to get underway with German U-Boats attacking Allied shipping, costing many Allied sailors lives and the loss of important war material. It was during this period that, on 14 October 1939, the British battleship HMS Royal Oak was sunk at Scapa flow by the German U-Boat U47 commanded by Günther Prien, with the loss of 833 out of a complement of 1,234. On 3 September 1939 the ocean liner 'Athenia' was torpedoed and sank. Of the 1417 passengers and crew 112 died. To the people at sea there was no Phoney war, it was a vary real and brutal conflict that brought death and misery to many.
The British conducted a night bombing campaign at this time, but the payload was propaganda leaflets rather than high explosives. The campaign was seriously criticised but many in the government believed it to be useful.
The British enforced a nightly blackout at home. This had serious consequences, with road accidents at night rising sharply. Low density lighting was introduced in an attempt to reduce the accidents but this was only allowed over a distance of 12 miles from the south-east coast. During January 1940 new car headlamps were introduced and a 20 mph speed limit was imposed in built up areas.
The lack of military activity on the continent lulled people into a false sense of security; in Britain many of the evacuees, who had moved away from the urban areas at the outbreak of war, returned home, people stopped carrying their gasmasks and many decided to go on holiday ignoring the war altogether. In France many of the conscripts would return from the front for the weekend on unofficial 'leave'.
On 9 April 1940 Germany launched its offensive against Denmark and Norway thus ending the 'Phoney War'.