When Rommel became aware of the strength of the place he realised that his troops helter-skeltering towards the town might dash themselves against the fortress and be destroyed piecemeal. He determined to throw an iron ring around the town quickly and intercepted the reconnaissance battalion and the anti-tank battalion as they headed eastwards. He switched them southwards and ordered them to pass round the British positions and to try to find a weak spot on the flanks. East of Acroma, artillery fire and extensive barbed wire defences brought the advance by the German troops to a halt. Rommel then gave orders to General Streich to swing further south and to advance upon El Adem. At that point there was a road up which his forces could drive and thereby enter Tobruk from the south. Streich pivoted his troops behind Brescia and moved to carry out his orders but sand storms and the usual inaccurate maps, together with poor navigation, so delayed his advance that he did not reach his objective until 11th. Local successes were scored against British patrols throughout the day but the main intent — to advance up the southern road - failed and the newly arrived panzer regiment, which then tried to force a passage on 12th, was brought to a halt by gun fire and an anti-tank ditch.
The first of the phases of the battle of Tobruk had opened. Rommel next changed the direction of his attack and flung a machine gun battalion to the east of Tobruk but that assault, too, failed in the face of heavy and accurate artillery fire which the defenders summoned up to defeat the Axis assault.
German troops had now thrown a cordon around the town and, although they were still too weak to carry out a major assault, it was felt that they had sufficient strength to defeat any break-out attempt by the garrison. The danger lay more in an offensive to raise the siege which British forces outside Tobruk might make.
The 8th Army had a screen of weak but mobile columns operating around, or holding certain strategic areas to the east of Tobruk, notably at Halfaya Pass, Sollum, Bardia, and Sidi Barani. This open eastern flank now became Rommel's immediate worry for it was from the east, along the Via Balbia and the desert tracks that the Matildas and the other British armoured fighting vehicles would storm in a massive counter-offensive. It was important that, even if the Axis forces did not have the strength to defeat an all-out blow, the British advance should be obstructed for as long as possible and their movements impeded. To achieve this it was essential to seize and to hold the ground running south from Bardia to the Halfaya Pass. If this area could be secured then the British forces attempting to move westwards could not use the Via Balbia but would be condemned to a wide and fuel-consuming drive via Sidi Omar. Such a deflection would not only break up their concentrations and betray the direction and strength of their thrust, but also allow time to prepare against it.
In order to know the significance of this eastern sector its topography must be understood. From the town of Sollum an escarpment, generally impassable to vehicles and difficult even for infantry, runs inland on a south-east line for about 50 miles. Only at Sollum and at Halfaya, 8 miles to the south-east of that place, were there passes permitting the escarpment to be crossed without difficulty and it was for the possession of those passes that the battles of the eastern flank were to be fought.
So that the reader is not confused with the fighting which then went on at the western flank around Tobruk and that which was conducted on the eastern front around Halfaya, this latter will be recounted separately, for the battles were, in a sense, only the seizure and consolidation of an outpost and, in that sense, a subsidiary to the main effort which was being made around Tobruk.
Rommel then removed from his beleaguring army a small detachment and sent this in a thrust to the Egyptian frontier with orders to take the vital positions in the Sollum—Halfaya area. The battle group was to act as a blocking force and as an outpost,from which would come early warning of any relief operation against Tobruk. On 13 April, the specially reinforced 15th Motor Cycle Battalion and 3rd Reconnaissance Battalion reached the frontier and went on to capture Bardia, Fort Capuzzo, and Sollum from the weak British garrison which then withdrew into Egypt. But 8th Army still held Halfaya Pass and the Germans, too few in numbers to force the issue, made no serious effort to take the pass but contented themselves with consolidating the gains which they had made. On the following day, the 15th Panzer Division was entrusted with control of operations along the frontier and took command of all Axis troops in the area, for now one of Trento Division's motorised battalions, Montemurro, had been brought forward and placed in reserve.
Weak British counter-attacks to capture the lost ground were launched on 15 April and during the following days they became more frequent, larger in number, and more difficult to beat back. The Axis forces were hard pressed and were being forced back under pressure from British armour and infantry, from the Royal Air Force which attacked their positions with low level machine gun attacks and high level bombing, and from the Royal Navy which took part in the operation by sending a monitor to bombard the positions in Sollum. But then on 17th came reinforcements with the arrival of a mixed flak battalion whose two light and three heavy batteries were soon in action against British tanks. Stukas of the Luftwaffe, sent into aid the embattled ground forces, dive-bombed and sank the monitor. Rommel was determined that the eastern enclave would hold.
Fighting flared or sank until 26 April, with the German and Italian troops maintaining their positions but on short commons because their ration truck convoys were intercepted and destroyed by British armoured car patrols. On 24 April the 15th Panzer Division decided that its forces were strong enough to strike for Halfaya and, aware that the British tankmen frequently mistook the Volkswagen cars for Panzer I, bluffed their opponents as to the actual number of armoured fighting vehicles which were under command.
Rommel had been informed of the forthcoming attack and, anticipating victory, had already issued orders that elements from 15th Panzer would be withdrawn to the Tobruk front once Halfaya Pass was safely in German hands. The vehicles for the assault formed up ready to move forward but then came disaster as the Luftwaffe bombed its own troops and caused a postponement of the attack until 26th. To replace the losses and to strengthen the assault the positions held by German units north of Capuzzo and in Sollum were taken over by the Italians and Rommel's men were disposed on either side of the fort. At top speed the advance roared forward into the pass but was halted abruptly by extensive and sophisticated mine-fields. Against fanatical resistance the German point units dawed their way, until by nightfall they had captured the British trench line. Immediate and heavy thrusts by 8th Army's armour flung back the main body of 15th Panzer Division to Capuzzo leaving only infantry outposts to hold the Pass. During the night of 26/27th reinforcements from 3rd Reconnaissance Battalion, the Montemurro Battalion, and a German motor cycle company were sent up to thicken the line and the local commander now considered that he had sufficient strength to force the pass. The elements of 15th Panzer were returned to Tobruk to help in the assaults on that town, while a battle group struck again into Halfaya pass and overran the British defenders who withdrew south and east to the line Buk Buk-Sofafi. The pass had been captured and with the positions consolidated the motor cycle and the reconnaissance battalions returned to the Sollum sector and passed into reserve. The eastern flank was firm and a German enclave, although isolated from the main body of the panzer group and surrounded by British forces, was firmly in position along the Egyptian border. To the west of the outpost was the mass of the Axis forces investing a British enclave at Tobruk.
On 14 April Rommel had decided that he would assault Tobruk frontally with the forces at his disposal, hoping to catch the defenders not fully prepared. Against a garrison of unknown strength he could pit only 100 tanks, a machine gun battalion, an artillery battalion, an engineer company, and elements from Ariete Division, together with parts of an anti-tank battalion.