The Italians had fortified Tobruk, their only naval base in Eastern Cyrenaica, before the war but after being routed at the Attack on Nibeiwa, the Battle of Sidi Barrani and the Battle of Bardia the Italian 10th Army had lost eight of its nine divisions and had only the 61st Infantry Division "Sirte" and stragglers to defend the port.

Further units were four light tank battalions, remnants of the "Catanzaro" division, and the XXI Corps Artillery Regiment.

After the encirclement of Tobruk, Graziani informed Mussolini that This morning the investment of the position by enemy armoured vehicles has begun.

(Stamani si è iniziato l'investimento della Piazza da parte dei mezzi corazzati nemici.

Dopo di che i vari episodi del nuovo dramma sono facilmente prevedibili.

)Graziani informed Mussolini that the 34 mi (54 km)-long Tobruk perimeter was manned by only 22,000 men with 340 guns, a number wholly inadequate for the task.

[5] On 9 January, Graziani informed the garrison commander, General Enrico Pitassi Mannella, that there would be no attempt at relief.

[7] Although Pitassi Mannella had thirty-two L3/35 tankettes and thirty-nine M11/39 tanks, only seven of the latter were operational and in three weeks of attempts to repair the M11/39s only three were serviceable enough to move in an engagement.

Pitassi Mannella had received no spares or fuel for the tanks and had the lightly armed and thinly armoured L3/35 and the M11/39s buried in the sand as strong points.

[8] The first line of the Eastern Sector was manned by the troops of the Guardia alla Frontiera reinforced with four companies from the 69th Infantry Regiment.

Expecting the main attack from this direction, Pitassi Mannella established a second line of defence 2–4 mi (4–6 km) behind the strong points, based on a small hill at the junction of the El Adem and Bardia roads.

To make up for the lack of anti-tank mines Pitassi Mannella had 2,200 26 lb (12 kg) bombs and 800 33 lb (15 kg) bombs, left behind by the Regia Aeronautica (Royal Italian Air Force), buried upright in the desert, in the hope that a British tank passing over them would trigger the impact fuze.

[9] After surrounding Tobruk, the WDF had exhausted the ample Italian supplies captured at Capuzzo and Sollum; O'Connor directed that the supplies flowing through the port of Sollum (350 long tons (356 t) per day in early January and 500 long tons (508 t) daily late in the month) to the 10th and 11th Field Depots he had set up about 43 mi (70 km) east of Tobruk.

On 19 January the Royal Air Force (RAF) dropped leaflets calling on the Italians to surrender but Pitassi Mannella took no notice.

[12] From midnight to 2:00 a.m. on 21 January the Royal Navy monitor HMS Terror and three smaller ships bombarded Tobruk, while destroyers waited further out to attack San Giorgio, if the crew tried to escape.

[19] At 1:00 p.m., Pitassi Mannella ordered the mobile reserve, with the seven operational M11/39s, to attack the Australian left flank from behind an artillery barrage.

[18] At the same time the 6th Divisional Cavalry Regiment had reached the outskirts of Tobruk but then been stopped by fire from San Giorgio.

Having lost contact with forces outside of Tobruk, Admiral Massimiliano Vietina organised the defence of the harbour with the few men at his disposal.

[18] By nightfall half of the Tobruk fortified area had been captured and at 4:15 a.m. on 22 January, Vietina ordered Captain Stefano Pugliese to blow up the magazines of San Giorgio to deny it to the British.

[19] Most of the demolitions had been of stores rather than installations; the Inshore Squadron of the Royal Navy began mine sweeping immediately and opened the port on 24 January.