This was a British unrealised plan derived from ‘Hammer’ (i) but limited to the capture and/or the neutralisation of the forts at the mouth of the Trondheimfjord in western Norway (26 April 1940).
To bolster the Allied effort in central Norway as the German forces surged north after their ‘Weserübung’ landings, Admiral Sir Dudley Pound, the First Sea Lord, revived the ‘Hammer’ concept for a naval bombardment of the German positions in the Trondheim area to divert German attention at a critical stage in the development of the ‘Maurice’ (ii) and ‘Sickle’ (i) pincer movement in central Norway. On 23 April, therefore, it was proposed that a British-led undertaking should capture or neutralise the forts at the mouth of the Trondheimsfjord, thereby securing command of the waters between Trondheim and Steinkjer, and so securing the flank for a renewed advance from Namsos; a small force might also be landed to help.
This was the germ of ‘Hammer II’, which the chiefs-of-staff outlined as a sequence of events in which naval bombardment neutralised the forts, which would then be captured by a force of two battalions supported by four howitzers disembarked from landing craft and paddle steamers. The assault on the forts from the land side was advocated by Norwegian officers who had escaped from the area and been interviewed at Namsos. These officers were unaware, however, that the Germans had strengthened the defences by bringing in mortars. Once Agdenes was in British hands, any German ships in the fjord would be destroyed and the British would land troops ‘at points to be decided upon’ to dislodge the German garrison of Trondheim.
The entire plan was dependent, however, on the withdrawal from France and redeployment of two infantry brigades of the regular army: their extraction from France would impose a 10-day delay in the implementation of ‘Hammer II’. It was accepted that the plan was ‘somewhat hazardous’, and should it succeed still left the problem of how to hold Trondheim. This would require the delivery and sustenance of numbers of aircraft, anti-aircraft guns, naval escorts, and shipping than the British currently possessed.
So after it had been considered by the chiefs-of-staff on 25 April, when the Prime Minister Neville Chamberlain gave preliminary orders through Winston Churchill, the First Lord of the Admiralty, for the start of preparations, at 10.00 on the following day the Military Co-ordination Committee decided to abandon it for lack of the air resources both for the assault and the protection of the port when taken.
The committee also decided that central Norway should be evacuated.