'Chronometer' was the British capture of airfields and other Italian installations at Assab, the main port of Eritrea, an Italian possession on the south-western coast of the Red Sea (10/15 June 1941).

On 26 May 1941 Rear Admiral R. H. C. Hallifax, senior officer of the British Red Sea Force, and the senior RAF officer in Aden reached Harar to meet Lieutenant General A. G. Cunningham, commanding the East African Force. In discussing the possibility of taking Assab, Cunningham agreed with Vice Admiral R. Leatham, the naval commander-in-chief in the East Indies that the capture of this port was desirable to halt the flow of Italian supplies being sent to Djibouti and to check intrigues which the Italians from Assab were fomenting in Yemen. Cunningham also agreed to return to Aden the 3/15^th Punjab Regiment.

A planning committee with the representatives of the three services was then established to create 'Chronometer'. On returning from Harar, Hallifax took the opportunity to overfly Assab, and this and other reconnaissances confirmed that all the coast-defence guns had apparently been removed and that the place was held only lightly.

The light anti-aircraft light cruiser Dido, which was due to call at Aden on her way for repair to damage caused by bombs hits in the Mediterranean, was placed at Hallifax’s temporary disposal, and the operation was planned for dawn on 10 June 1941. Hallifax departed Aden in Dido on 9 June, carrying with him Brigadier W. A. Dimoline, commanding the land forces. The Indian sloops Indus and Clive were tasked with carrying out an exploratory sweep before the start of the main operation.

Strong headwinds delayed Clive, which was towing the tug Kalisco, and the transport Tuna, which with the armed boarding vessel Chakdina was carrying motor transport and other important stores as well as the 3/15^th Punjab Regiment, one company of signallers and one company of Royal Engineers.

On reaching Perim island, at the extreme south of the Red Sea in the Bab el Mandeb between Yemen and Djibouti, Hallifax and Dimoline decided to postpone the operation by 24 hours, and Hallifax therefore ordered the force to enter Perim harbour and anchor. The postponement enabled the 3/15^th Punjabs, who had been packed into Chakdina for 60 hours since leaving Berbera, to have some time ashore. Hallifax also decided to leave Kalisco at Perim as, in view of the weather, it seemed improbable that she would arrive at Assab. He therefore arranged to remove her minesweeping gear and hoped that if inshore sweeping were necessary he would be able to use a local tug which he knew was at Assab and which he hoped to capture intact.

The expedition left Perim at 19.30 on 10 June and headed to the north in two columns. There was a fairly strong northerly wind and Hallifax decided to amend the position already laid down in his orders so as to avoid large alterations of course and to pass either within visual distance of Harbi island, or at any rate a range which would give accurate radar results. On the run to the north the instrument proved to be of the greatest value and an accurate range was obtained of the Hanish islands and later Harbi and Sanah Bor islands, which enabled the expedition to approach and anchor, as previously arranged, with great accuracy.

On the night of 10/11 June the force approached the Assab anchorage from the north, Indus using double Oropesa sweeping ahead to a position off the port. The expedition was swept into the anchorage by Indus, but no mines were encountered. On anchoring at 03.30, Indus carried out searching sweeps toward Sanah Bor and round the position where Dido was anchored to ensure that the ships had the room to manoeuvre safely should this become necessary, and then waited close to Chakdina to sweep her into the inner harbour when ordered. At 04.34 the two motor boats, each carrying 30 men of the 3/15 Punjabs, moved into the harbour passing close eastward of Sanah Bor island.

Between 05.05 and 05.12 Dido executed carefully planned shore bombardment, and at the same time aircraft appeared over the town and bombed the roads in the approaches and flew low over the town to drown the noise of the motor boats approaching: in all, eight attacks were undertaken by the RAF’s Nos 8 and 39 Squadrons, which also destroyed a significant number of Italian aircraft on the ground. At 05.19 the motor boats arrived off the main pier and landed the soldiers without difficulty or opposition. The senior officer also seized the Italian tug before it could be scuttled.

On the Italian side there was effectively no sign of alertness, and the bombardment caught many of the Italians asleep on the pier. Two Italian generals were captured asleep in their beds. At 06.00 the signal indicating success was seen in Dido, and Hallifax ordered Chakdina and then Tuna to enter the harbour, Indus sweeping in ahead of them and Clive marking the channel. No mines were encountered, and Chakdina was anchored off the pier and the remainder of the 3/15^th Punjabs immediately landed. Tuna was berthed alongside at a partially completed jetty close to the southward pier and disembarkation began here too.

At 07.00 the Italian civil governor and an interpreter reached Dido by motor boat and surrendered the town of Assab to Hallifax and Dimoline. Indus then swept out and buoyed a channel into the harbour, and at 11.00 Dido entered and anchored off Ras Gombo lighthouse. During the afternoon Dido despatched parties to examine the islands, but no personnel or matériel were discovered.

Capitano di Vascello Bollo, the senior Italian naval officer, was captured and provided information about three minefields. On the next day the ships embarked 547 prisoners including two generals, Bollo and 35 Germans. On 13 June, two days after Assab’s capture, the Indian trawler Parvati became the last naval casualty of the campaign when she struck a magnetic mine near Assab.