This was a British carrierborne air attack on the coastal town of Elmas and the airfield at Cagliari on the Italian island of Sardinia within the context of ‘Hats’ to run reinforcements through the Mediterranean for Admiral Sir Andrew Cunningham’s Mediterranean Fleet, and at the same time to provide a diversion from the primary undertaking (1/2 September 1940).
The ‘Hats’ force comprised Force ‘B’ (the reinforcement for the Mediterranean Fleet) and the whole of Vice Admiral Sir James Somerville’s Gibraltar-based Force ‘H’, comprising the battle-cruiser Renown, fleet carrier Ark Royal, light cruiser Sheffield, and 17 destroyers of the 8th and 13th Destroyer Flotillas, with the exception of the unmodernised battleship Resolution, which was deemed too slow for involvement.
It was at 08.45 on 30 August that Somerville’s forces and the reinforcement for the Mediterranean Fleet departed Gibraltar. Somerville’s forces steered a zigzagging anti-submarine course to the east at 15.5 kt. In essence, the 'Hats' plan was that the ships of Force 'H' should accompany Force 'B' as far as a position to the south-east of Sardinia before turning back to Gibraltar, whereupon the reinforcements would steam through the Sicilian Narrows to a rendezvous with the Mediterranean Fleet.
Except for the destruction of two Italian reconnaissance floatplanes by Blackburn Skua aircraft from Ark Royal, 31 August passed without incident. At 21.50 the destroyers Velox and Wishart were detached to carry out 'Squawk', another carefully designed feint: to the north of the Balearic islands group and still steering to the north-east, the two destroyers transmitted a series of radio signals during the night intended to mislead the Italians into thinking that Somerville’s force was steaming to the north-east in the direction of the Gulf of Genoa. The transmissions also to serve the secondary purpose of covering Ark Royal's low-power signals flying operations during the next phase of 'Hats', which comprised the 'Smash' and 'Grab' air attacks on Cagliari.
At 22.00, Somerville’s ships altered course, without signal, from north-east to south-east. At 03.25 on 1 September, at a distance of 115 miles (185 km) from Cagliari, the ships altered course in order to fly off the attack force of nine Fairey Swordfish bombers each carrying four 250-lb (113-kg) general-purpose bombs and eight 25-lb (11.3-kg) incendiary bombs. The aircraft took up formation over flame floats dropped some 10 miles (16 km) from the fleet, and flew to Cagliari through clear weather. At 06.00 the aircraft dropped their bombs on the installations at Elmas airfield, illuminated by parachute flares, from an altitude of only 300 ft (90 m), and as they turned back toward their carrier the aircrews could see fires behind them. By 08.00 all the Swordfish aircraft had landed on the carrier.
Somerville now ordered an alteration of course to the south-west with the object of deceiving the Italians into believing that the main objective of the British sortie from Gibraltar had been to attack Cagliari, and that the British ships were now heading back toward harbour. However, as Somerville later reported, 'As the Force was apparently not being shadowed at this time, it is probable that this ruse failed.'
At 10.30 the ships again changed onto a course slightly to the north of east, and from this time the ships were covered by a pair of fighter patrols, each of six aircraft, as they were now steering into the area of maximum danger from Italian aircraft. Even so, the day passed without incident except for a mistake in which Fairey Fulmar fighters from Illustrious, one of the reinforcement ships for the Mediterranean Fleet, attacked a Lockheed Hudson patrol aeroplane.
At 22.00 on 1 September, at a position about half way between the south-eastern tip of Sardinia and the western tip of Sicily, the Force 'F' reinforcements for Cunningham parted company in the dark without signal and set course to the south-east.
With his primary mission now accomplished, Somerville next turned to the north and then to the west, increasing speed to 24 kt. In the dark early in the morning of 2 September, Ark Royal's aircraft again attacked Cagliari: there were again no losses, but also no success as haze and low cloud obscured the target. At 08.00 Somerville ordered an increase in speed to 26 kt and a course to the west, and the ships of Force 'H' reached Gibraltar during the morning of 3 September without coming under attack by Italian aircraft despite the fact that they had been within effective range of Italian air bases for at least 48 hours. The fact was a considerable surprise to Somerville and other senior officers of Force 'H', who had prepared a welcome in the form of combat air patrols and a plan for closely concentrated anti-aircraft fire which, it had been hoped, would 'deliver a blow to the Italian Air Force which might have a telling and lasting effect'.