The Allies tightened the economic blockade on Germany; blocking the supply of Swedish iron ore via Norway was considered particularly galling for the Germans. In April 1940 the British and French took action to prevent the Germans gaining access to Swedish iron ore via the Norwegian port of Narvick. By sheer coincidence the Germans decided to invade Denmark and Norway at the same time. The Germans moved more quickly than the Allies and Denmark was overrun in a day. Despite Norwegian fighting, the Germans soon established themselves around Oslo, Trondheim, Bergen and Narvick, and seized airfields from where they operated their own aircraft.
The Cabinet were forced to change their thinking with regards to acceptable ways of waging modern warfare. British submarines were now authorised to carry out unrestricted submarine warfare in the waters around Denmark and Norway. The RAF and Fleet Air Arm were also allowed to carry out attacks on any shipping found in those areas.
On 10 May 1940 Germany launched its attack on France. On the very same day Neville Chamberlain's leadership suffered a crisis in confidence and Winston Churchill was asked to form a national coalition government. Belgium and Holland asked for Allied help, and so the northern group of armies, including the BEF. moved to resist the German advance through the Low Countries. However, a heavy blow fell on Luxemburg and the Ardennes forest, and the Germans shattered the French Army around Sedan. German armoured divisions poured into France and raced across to the channel coast near Abbeville, cutting off the British and French forces in Belgium and Northern France.
Paralysed by the speed of the German advance, the French Army never recovered. Politicians in France and Britain were aghast at the speed of the collapse. In the north, the British were forced to retreat to Dunkirk where the Navy succeeded in rescuing many of the troops. The technique was then repeated in ports along the French Atlantic coast. Losses were heavy, but hundreds of thousands of Allied troops were brought back to Britain.
The British Expeditionary Force had left almost all its equipment behind on the beaches of Dunkirk and was in bad shape. By early June it was clear that France was now incapable of offering organised resistance to the Germans. The War Cabinet rejected the idea of seeking terms if France surrendered. By the time France finally collapsed, the decision had been made - Britain would fight on.