At the Battle of Tug Argan (11–15 August) Italian attacks had the advantage of artillery and the outnumbered defenders were gradually worn down and slowly outflanked, until the remaining fortified hilltops were vulnerable to capture.

After the failure of a counter-attack towards the Mirgo Pass, the local commander, Major-General Reade Godwin-Austen, had too few men to retrieve the situation and to keep open an escape route at the same time and was given permission to retreat towards Berbera.

[1] On 10 June 1940, Mussolini declared war against Britain and France, making the Italian forces in Africa a threat to British supply routes along the Mediterranean, the Suez Canal and the Red Sea.

[3] The small British and Commonwealth force garrisoned the Suez Canal and the Red Sea route, which was vital to Britain's communications with its Far Eastern and Indian Ocean territories.

Faced with frontiers guarded by about eight men to the mile, Wavell concluded that a defensive strategy was the only feasible policy and intended to mount delaying actions at the main posts and hope for the best.

Italian apprehension about the possibility of the British using the colony as a bridgehead continued; Germain was superseded by Pierre Nouailhetas as governor beginning on 7 August, who arrived from France on 2 September.

According to the British military commander, Brigadier Arthur Chater RM, a small reinforcement would give the local garrison the means to resist an invasion of the colony for twelve days, sufficient for a relief force to arrive from India, but the policy was rejected.

The main road into Somaliland from Djibouti was commanded by hills with six passes good enough for wheeled vehicles; both sides agreed that they must be garrisoned to deny them to the Italians and provide a base for debouching when an Allied counter-offensive began.

[13][11] After the Armistice of Villa Incisa, the end of pro-British prevarication by Legentilhomme led to Chater receiving orders to plan an evacuation if the colony became untenable.

[15] The British expected that Berbera would be the objective of an Italian invasion; the frontier with Ethiopia was too long to guard and there was no position from whence to contest the approaches to the port through Zeila near French Somaliland, thence east along the coast road or through Hargeisa or via Burao.

While waiting, Aosta and his deputy, General Guglielmo Nasi, completed an appreciation of the likely opposition and the campaign objectives; on 14 July they predicted that the main battle would be fought in the Karim and Jerato passes; if the defenders stood their ground, the Italian troops would be able to envelop their flanks.

[18] The Italian invasion force included five colonial brigades, three Blackshirt battalions and five Bande, half a company of M11/39 medium tanks and a squadron of L3/35 tankettes, several armoured cars, 21 howitzer batteries, pack artillery and air support.

[21] Because the Assa Hills rose to over 4,500 ft (1,400 m), parallel to the coast about 50 mi (80 km) inland, there were three approaches to Berbera for wheeled and tracked vehicles for the Italians to consider.

[20] The direct route with the widest pass was via Hargeisa and the Italian plan was for the western column to seal off French Somaliland and then send light forces east along the coast road.

The western column (Lieutenant-General Sisto Bertoldi) was to advance towards Zeila to seal the border with French Somaliland and then move eastwards along the coast road towards Berbera.

On 1 August Comando Tattico dell Settore Aeronautica Ovest (CTSAO, Western Sector Tactical Command) was created for the invasion of British Somaliland.

On 4 August, two SM 79 bombers of 44° Gruppo arrived at Dire Dawa and two of the 94 Squadron Gloster Gladiators at Berbera transferred to Laferug further south closer to Tug Argan, both strips having only small-arms for defence.

[27] At 01:00 twelve light tanks advanced in line; three were hit and knocked out by Boys anti-tank rifle fire from the SCC and the company of the NRR defending the hill station.

The defenders reported the presence of Italian medium tanks and the captain of HMAS Hobart donated the ship's QF 3-pounder Hotchkiss saluting gun, a three-man crew and 30 rounds of ammunition.

[29] In the north, the Bertoldi column captured Zeila, about 150 mi (240 km) north-west of Berbera, cutting communications with French Somaliland and then began a slow advance south-east along the coast road, under intermittent air attack from Aden and bombardment from the sea, pushing back the SCC rearguards as far as the village of Bulhar by 17 August.

[31][20][c] Just before Wavell left Cairo to visit London for talks, reports of the size of the invasion force led him to order most of a field artillery regiment and two anti-tank guns of the 4th Indian Infantry Division to be sent from Egypt to Somaliland by special convoy.

[37] On 13 August the aircraft of CTSAO concentrated on the area to the east of Adadleh; the Jerato Pass was bombed by Ca 233s and the crashed Blenheims at Berbera were strafed twice by CR 32s, one on the second attack being hit by small-arms fire and landing inside British-held territory.

Godwin-Austen informed Henry Wilson, the GOC-in-C British Troops in Egypt, of the situation, concluding that further resistance at Tug Argan would be futile and likely to result in the loss of the force.

The Black Watch, two companies of the 2nd KAR and elements of 1/2nd Punjab Regiment formed a rearguard at Barkasan on the Berbera road, about 10 mi (16 km) behind Tug Argan and other troops moved back to Nasiyeh.

[44] The Royal Navy had built an all-tide jetty and had commenced evacuating civilian and administrative officials; on 16 August, the British started to embark troops.

[46] On 17 August, the Italian western column at Bulhar, about 40 mi (64 km) west of Berbera, was engaged by the light cruiser Ceres and halted by gunfire.

[48] Before sailing for Aden early on 19 August, Hobart, with the force headquarters aboard, stayed behind to collect stragglers and complete the destruction of buildings, vehicles, fuel and stores.

[52] The Italians had shown the ability to coordinate columns separated by many miles of desert; the forces under British command had kept their discipline during the retreat and preserved most of their men.

The British learned that inadequately defended airstrips could quickly be made untenable, leaving bombers flying from Aden unprotected against air attack.

Delays caused by the terrain, weather and the cancellation of a coup de main by 300 Italian infantry on the port, had enabled the British to get away, despite the improvised nature of the embarkation.