On 3 August 1940, the Italians invaded with two colonial brigades, four cavalry squadrons, 24 M11/39 medium tanks and L3/35 tankettes, several armoured cars, 21 howitzer batteries, pack artillery and air support. The British had a garrison of two companies of the Sudan Defence Force, two motor machine-gun companies and a mounted infantry company. Kassala was bombed and then attacked, the British retiring slowly. On 4 August, the Italians advanced with a western column towards Zeila, a central column (Lieutenant-General Carlo De Simone) towards Hargeisa and an eastern column towards Odweina in the south. The SCC skirmished with the advancing Italians as the main British force slowly retired. On 5 August, the towns of Zeila and Hargeisa were captured, cutting off the British from French Somaliland. Odweina fell the following day and the Italian central and eastern columns joined. On 11 August, Major-General Alfred Reade Godwin-Austen was diverted to Berbera, en route to Kenya to take command as reinforcements increased the British garrison to five battalions. (From 5 to 19 August, RAF squadrons based at Aden flew 184 sorties, dropped 60 long tons (61 t) of bombs, lost seven aircraft destroyed and ten damaged.)
On 11 August, the Italians began the Battle of Tug Argan (tug, a dry, sandy riverbed), where the road from Hargeisa crosses the Assa hills and by 14 August, the British were at risk of defeat in detail by the larger Italian force and its greater quantity of artillery. Close to being cut off and with only one battalion left in reserve, Godwin-Austen contacted Henry Maitland Wilson, commander of the British Troops in Egypt in Cairo (Wavell was in London) and received permission to withdraw from the colony. The 2nd Battalion, Black Watch, supported by two companies of the 2nd King's African Rifles and parties of the 1st/2nd Punjab Regiment covered the retreat of the British contingent to Berbera.

By 2:00 p.m. on 18 August, most of the contingent had been evacuated to Aden but HMAS Hobart and the HQ stayed behind until morning before sailing and the Italians entered Berbera on the evening of 19 August. In the final four days, the RAF flew twelve reconnaissance and 19 reconnaissance-bombing sorties, with 72 attacks on Italian transport and troop columns; 36 fighter sorties were flown over Berbera. The British suffered casualties of 38 killed and 222 wounded; the Italians suffered 2,052 casualties; fuel and ammunition expenditure and wear and tear on vehicles was difficult to remedy, which forced the Italians to return to the defensive. Churchill criticised Wavell for abandoning the colony without enough fighting but Wavell called it a textbook withdrawal in the face of superior numbers.
Anglo-Egyptian Sudan shared a 1,000 mi (1,600 km) border with the AOI and on 4 July 1940, was invaded by an Italian force of about 6,500 men from Eritrea, which advanced on a r…

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On the Italian declaration of war on 10 June 1940, East Africa Force (Lieutenant-General Douglas Dickinson) comprised two East African brigades of the King's African Rifles organised as a Northern Brigade and a Southern Brigade with a reconnaissance regiment, a light artillery battery and the 22nd Mountain Battery Royal Indian Artillery (RIA). By March 1940, the KAR strength had reached 883 officers, 1,374 non-commissioned officers and 20,026 African other ranks. Wavell ordered Dickinson to defend Kenya and to pin down as many Italian troops as possible. Dickinson planned to defend Mombasa with the 1st East African Infantry Brigade and to deny a crossing of the Tana River and the fresh water at Wajir with the 2nd East African Infantry Brigade.

Detachments were to be placed at Marsabit, Moyale and at Turkana near Lake Rudolf (now Lake Turkana), an arc 850 mi (1,370 km) long. The Italians were thought to have troops at Kismayo, Mogadishu, Dolo, Moyale and Yavello, which turned out to be colonial troops and bande, with two brigades at Jimma, ready to reinforce Moyale or attack Lake Rudolf and then invade Uganda. By the end of July, the 3rd East African Infantry Brigade and the 6th East African Infantry Brigade had been formed. A Coastal Division and a Northern Frontier District Division had been planned but then the 11th (African) Division and the 12th (African) Division were created instead.

On 1 June, the first South African unit arrived at the port of Mombasa in Kenya and by the end of July, the 1st South African Infantry Brigade Group had arrived. On 13 August, the 1st South African Division was formed and by the end of 1940, about 27,000 South Africans were in East Africa, in the 1st South African Division, the 11th (African) Division and the 12th (African) Division. Each South African brigade group consisted of three rifle battalions, an armoured car company and signal, engineer and medical units. By July, under the terms of a war contingency plan, the 2nd (West Africa) Infantry Brigade, from the Gold Coast (Ghana) and the 1st (West Africa) Infantry Brigade from Nigeria, were provided for service in Kenya by the Royal West African Frontier Force (RWAFF). The 1st (West African) Brigade, the two KAR brigades and some South African units, formed the 11th (African) Division. The 12th (African) Division had a similar formation with the 2nd (West African) Brigade.

At dawn on 17 June, the Rhodesians supported a raid by the SDF on the Italian desert outpost of El Wak in Italian Somaliland about 90 mi (140 km) north-east of Wajir. The Rhodesians bombed and burnt down thatched mud huts and generally harassed Italian troops. Since the main fighting at that time was against Italian advances towards Moyale in Kenya, the Rhod…

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Operation Camilla was a deception concocted by Lieutenant-Colonel Dudley Clarke to deceive the Italians, making them believe that the British planned to re-conquer British Somaliland with the 4th and 5th Indian divisions, transferred from Egypt to Gedaref and Port Sudan. In December 1940, Clarke constructed a model operation for Italian military intelligence to discover and set up administration offices at Aden. Clarke arranged for the Italian defences around Berbera to be softened up by air and sea raids from Aden and distributed maps and pamphlets on the climate, geography and population of British Somaliland.

"Sibs" (sibilare, hisses or whistles), were circulated among civilians in Egypt. Bogus information was planted on the Japanese consul at Port Said and indiscreet wireless messages were transmitted. The operation began on 19 December 1940 to mature early in January 1941. The deception was a success but the plot backfired when the Italians began to evacuate British Somaliland instead of sending reinforcements. Troops were sent north into Eritrea, where the real attack was coming, instead of to the east. Part of the deception, with misleading wireless transmissions, did convince the Italians that two Australian divisions were in Kenya, which led the Italians to reinforce the wrong area.
In November 1940, Gazelle Force operated from the Gash river delta against Italian advanced posts around Kassala on the Ethiopian plateau, where hill ranges from 2,000–3,000 ft (610–910 m) bound wide valleys and the rainfall makes the area malarial from July to October. On 11 December, Wavell ordered the 4th Indian Division to withdraw from Operation Compass in the Western Desert and move to Sudan. The transfer took until early January 1941 and Platt intended to begin the offensive on the northern front on 8 February, with a pincer attack on Kassala, by the 4th and 5th Indian divisions, less a brigade each.

News of the Italian disaster in Egypt, the harassment by Gazelle Force and the activities of Mission 101 in Ethiopia, led to the Italians withdrawing their northern flank to Keru and Wachai and then on 18 January to retreat hurriedly from Kassala and Tessenei, the triangle of Keru, Biscia and Aicota. Wavell had ordered Platt to advance the offensive from March to 9 February and then to 19 January, when it seemed that Italian morale was crumbling. The withdrawal led Wavell to order a pursuit and the troops arriving at Port Sudan (Briggs Force) to attack at Karora and advance parallel to the coast, to meet the forces coming from the west.
Two roads joined at Agordat and went through to Keren, the only route to Asmara. The 4th Indian Division was sent 40 mi (64 km) along the road to Sabderat and Wachai, thence as fa…

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