'Hammer' (i) was a British unrealised plan derived from 'Boots' for the recapture of Trondheim on the west coast of Norway by means of a frontal assault in the immediate aftermath of the 'Weserübung' German invasion of that country (16/19 April 1940).

Assessed by the Germans as 'the pivot of all [Norwegian] operations', Trondheim had fallen to the 138^th Gebirgsjägerregiment of Generalleutnant Valentin Feurstein’s 2^nd Gebirgsdivision, a formation delivered by the ships of Gruppe II (the heavy cruiser Admiral Hipper and four destroyers of Fregattenkapitän Rudolf von Pufendorf’s 2^nd Zerstörer-Flottille) in 'Wildente' on 9 April.

In the surrounding areas Generalmajor Jacob Ager Laurantzon’s Norwegian 5^th Division was managing to check the German advances to the north, east and west, but it was appreciated by the Allied high command that the arrival of men from Generaloberst Nikolaus von Falkenhorst’s Gruppe XXI (Generalmajor Erwin Engelbrecht’s 163^rd Division and Generalmajor Richard Pellengahr’s 196^th Division) from the south up the Østerdalen and Gudbrandsdalen from Oslo would undo the defence. Thus an Allied grip on central Norway could only be ensured by the delivery of fresh troops for the recapture of Trondheim, which would deny the 2^nd Gebirgsdivision any chance of receiving a seaborne reinforcement.

'Hammer' (i) was clearly dependent on the ability of the Royal Navy to escort the necessary troopships, and when this was seen to be too risky against the threat of German bombers of General Hans Geisler’s X Fliegerkorps operating from captured bases, Admiral Sir Charles Forbes undertook to examine the possibility that units of his Home Fleet could carry the troops.

As regards the military force for the Trondheim landing, it was initially suggested that a force of 10 battalions (five British regular and five French) would be required, but the units finally selected were two British brigades and two Canadian battalions: the former were the regulars of Brigadier H. E. F. Smyth’s 15^th Brigade and the territorials of Brigadier G. Lammie’s 147^th Brigade. Of these, the latter was to be embarked separately as a reserve, while the rest of the force, including one divisional and two brigade headquarters, constituted the assault force supplemented by a Royal Marine battery of 3.7-in (94-mm) howitzers.

The expedition was to be commanded by Major General F. E. Hotblack, who received his orders on 17 April but suffered a stroke during the evening of the same day. His successor was Major General H. P. M. Berney-Ficklin, latterly a brigadier commanding the 15^th Brigade, but his command was terminated by an air accident on the morning of 19 April and Berney-Ficklin was succeeded by Major General B. C. T. Paget.

Air operations against Trondheim were also planned. Air Marshal Sir Charles Portal’s RAF Bomber Command was ordered to increase its attacks on German-occupied airfields, and during 'Hammer' (i) to neutralise German air activity by maximum-strength attacks which, it was hoped, would amount to a full-squadron raid each night on Værnes, Sola and Fornebu. For a number of reasons, however, the main task in securing British air superiority over 'Hammer' (i) would devolve onto the warplanes of the Fleet Air Arm operating as an integral part of Forbes’s Home Fleet.

Late in the evening of 13 April, it was reported that the Royal Navy anticipated no difficulty in silencing the shore batteries outside Trondheim, and the service staffs were instructed to prepare their plans accordingly. On the afternoon of 15 April the Military Co-Ordination Committee was informed that the Royal Navy would be ready to launch the direct attack in about a week. On the following day Admiral Sir Dudley Pound, the First Sea Lord, said that another seven or eight days were needed to mount the operation, of which it was assumed that the Germans would have no more than a two-hour warning and therefore be unable to call in air reserves from Denmark and northern Germany.

Two fleet carriers would be on hand to launch 80 aircraft to deal with the German air forces in Norway, and warships would then close to deal with any German artillery fire from the land.

On 17 April the committee decided that 'Hammer' (i) would be launched on 22 April, but on the following day the date was postponed by two days to allow command changes after Hotblack’s stroke and also provide the troops with time for disembarkation practice. On the morning of 19 April agreement on the naval preparations was completed as the committee decided that the battleship Warspite should be withdrawn from the Narvik operations to take part.

It had been decided to avoid any bombardment of Trondheim itself, and the initial landing points would be beyond the city in the bay near Værnes and at Levanger.

The feasibility of the navy’s commitment to 'Hammer' (i) was the subject of much discussion over this period. Forbes was informed of 'Hammer' (i)early on 14 April, while he was still at sea off the Lofoten islands group in the battleship Rodney, with a request for his opinion as to the possibility of destroying or dominating the shore batteries so that transports could enter, and for particulars of the ships which would be needed. Replying at 12.00 on 14 April, Forbes said that the operation was impossible unless the government was prepared to face heavy losses in troops and transports as the Germans would have sufficient warning to mount a continuous bombing effort. Forbes also pointed out that none of his ships was carrying the HE bombardment shells which would be needed for their main armament.

The Admiralty instructed Forbes to think again, for large troopships would have to be brought into the danger zone somewhere, and it was planned that 'Hammer' (i) would be shielded to a certain extent by the incapacitation of Stavanger and Trondheim airfields, the former by RAF bombing followed by bombardment from the heavy cruiser Suffolk, and the latter by the attacks of naval aircraft and the warship bombardment. The Admiralty added that HE shell for 15-in (381-mm) guns had been ordered to Rosyth, and that the fleet carrier Furious and the ships of 1^st Cruiser Squadron would be employed.

After receiving particulars of the defences and confirmation that the Germans had seized the batteries at the entrance of the fjord, Forbes responded on 15 April that the the withdrawal of the 1^st Cruiser Squadron from Kirkenes and of Furious from Narvik was tactically unsound, and as an alternative suggested a force comprising the fleet carrier Glorious, the capital ships Renown, Valiant, Warspite, at least four light anti-aircraft cruisers, about 20 destroyers, and many landing craft.

Forbes added that his previous reply had been misunderstood: he did not 'anticipate any great difficulty from the naval side' provided the troops were transported in warships rather than transport vessels.

The Admiralty went ahead with its preparations, and on 19 April Valiant departed for Rosyth to ship the HE shells for the bombardment.

Meanwhile Forbes had returned to the bulk of his Home Fleet at Scapa Flow, and arrangements were being made for the entire military assault force to be embarked, according to Forbes’s wishes, in cruisers, destroyers and sloops, which would also carry the stores, howitzer battery, and six landing craft. The embarkation date was fixed for 21 April at Rosyth, from which the expedition would steam to Scapa Flow to collect the reserve brigade and rendezvous with the fleet carriers Ark Royal and Glorious. These last became available only on 23 April, so the attack was in effect put back to 26 April at earliest.

By now, though, a more cautious policy was coming to the fore in London, where the authorities were increasingly concerned to limit the risks to which the UK’s slender resources should be exposed. Of particular concern was the vulnerability of major warships to air attack. Suffolk had reached Scapa Flow on the morning of 18 April with her quarterdeck awash after bombarding Sola airfield near Stavanger with only limited success, but then herself come under nearly seven hours of attack from the air. Sent north from Stavanger to pursue what were thought to be German destroyers, the heavy cruiser could not be found by her intended RAF fighter escort, which had expected the ship to be closer inshore, and although Forbes despatched naval aircraft from Hatston in the Orkney islands group as well as both his battle-cruisers, this could not prevent a direct hit from a 1,102-lb (500-kg) bomb. There were four more German air attacks even after nine fighters had arrived to protect the cruiser.

Now there were also fears that, even if ships could be defended effectively by their own anti-aircraft guns and Fleet Air Arm fighters, the troops would be bombed as they went ashore to capture the city of Trondheim and the airfield at Værnes. By the morning of 20 April it had been decided to cancel 'Hammer' (i) and concentrate instead on the less ambitious combination of 'Maurice' (ii) and 'Sickle' (i).