The Croatian 7th Division was held back at Birhakim, and the Serbs fought so tenaciously that it was a tough bone to gnaw on. The assault on the area began before dawn on 18 May.

At 4:30 a.m., the darkest hour before dawn.

The silence of the desolate and silent Cyrenaica Desert was shattered by the rumbling of cannonballs, and clusters of fire rose up on the periphery of Bir Hakim, and shells drew a shining white line in the air and flew towards the enemy's forward positions. The Croatian 7th Division, concentrating almost all of its more than a hundred guns of 75 mm or more, launched a heavy artillery bombardment of the Serbian 3rd Division. The dense artillery fire soon turned the Serb positions into a sea of fire, and the ground trembled with a loud roar.

The shelling before the attack lasted more than an hour, and after a short pause, the roar of the motor was heard. In the first attack, Stett threw an armored battalion and an infantry battalion, and more than a hundred tanks and armored vehicles lined up in battle formation and began to launch a surprise attack on the enemy's positions. The number of artillery of the Serbian division was only about half that of the Croatian division, but the artillery was well concealed, and it was difficult for the artillery of the Croatian division to suppress them.

Enemy artillery fire continued to blow up the tanks, and several tanks and armored vehicles were destroyed, but many more continued to advance towards the tanks. Machine gun shells fired from Serbian positions hit the armor and sparked sparks, but they could not stop the advance of tanks and armoured vehicles.

Suddenly, a dull sound exploded from under the tank body, and the tracks of the tank were blown off, and it suddenly fell on the spot. As a result, more and more tanks were blown up, and the Serbs planted a large number of mines in front of their positions, and unfortunately these attacking armoured units plunged headlong into the minefields laid by the enemy.

In desperation, Stett had no choice but to give the order to stop the attack.

The first offensive of the Austro-Hungarian army ended in a crushing defeat, losing 27 tanks and armored vehicles in just half an hour. The crew of the tank escaped from the destroyed tank and retreated with the infantry accompanying the attack, but many fell on the road. In order to cover the retreat of the infantry and the remaining armored vehicles, the artillery on the Austro-Hungarian positions rang out again, and the Serbs' positions were filled with gunsmoke, and their fire was finally suppressed.

During the day, Stett ordered Melzin's Panzer Regiment to go around to the rear of the Serbian position and made two more attempts from the northeast, but the Serbs also laid a large number of mines in this direction, causing the Panzer Regiment to suffer considerable losses. At night, the 7th Infantry Battalion took advantage of the moonlight to break into the trenches of the Serb army's first line, but was driven out by the Serbs with bayonets, and the 7th Infantry Battalion, which was responsible for the assault, was wounded by nearly half. Stett was forced to halt the offensive and besieged instead, waiting for the engineer units to clear several passages in the minefield before the armored forces attacked.

An artillery battle ensued between the two sides at Birhakim, and the Croatian 7th Division was blocked on this line.

On the coastal front, however, the 21st Division advanced smoothly, and the British abandoned Derna and retreated to Tobruk with all their might.

General Birdwood, commander-in-chief of the British forces on the Cyrenaica front, appeared worried, and the Austrian fleet blocked the port of Tobruk, cutting off his sea lines of transport. Rear Admiral Weems engaged in a battle with the Austro-Hungarian fleet outside the harbor, trying to break through the enemy's naval blockade, but was beaten back. Two of the four British battleships in the harbor were now severely wounded and had to sit on the shore and use as defensive batteries.

Even more dangerous was the fact that yesterday evening an Austro-Hungarian cruiser fleet carried out an artillery bombardment of the port of Serum, where the British troops were weakly defended, and if the enemy landed from there, then the British retreat of the entire Cyrenaica would be cut off.

Outside the port of Tobruk, five Austro-Hungarian battleships and two armored cruisers, led by the "Empress Teresa", were shelling the port facilities and warships and transports in the harbor, and six British and French battleships were close to the coast and returned fire on the sea. With no fortified artillery, the Austrians approached as close as 15,000 yards and shelled the harbor.

Columns of water rose in the harbor, and the wharf also suffered great damage, and the enemy warships in the distance were blazing with guns in the rumbling sound, and clouds of white smoke rose on the sea. The Cornwallish returned fire with its four 12-inch guns, and the battered battleship ran aground on the beach near the entrance to the channel, becoming a key target.

The Indian 7th Division, which defended the Gazala line between Tobruk and Birhakim, quickly collapsed under the attack of the Austrians. The Indians, who had not seen tanks and did not know how to cope with armored weapons, scattered in front of tanks and armored vehicles. Although London repeatedly stressed the need to hold Tobruk, General Birdwood was not hopeful.

Hovald was plagued by a lack of troops, with the Croatian 7th Division pinned down at Birhakim and a 21st Division at his disposal, while the Marines were slowly preparing for a landing at Serum. The commander of the North African Corps then ordered Stett to draw an armored battalion and a mechanized infantry battalion from Bir Hakim and launch an attack on Baldije, behind Tobruk.

Sir Hamilton had no intention of following orders from London to defend Tobruk, putting four of his divisions in danger. After the attack on the port of Selum, the commander-in-chief of the British Middle East Army managed to get the approval of the London side to withdraw his troops from Tobruk.

On 23 May, the Austro-Hungarian Marines began landing at Serum, which prompted Hamilton to make up his mind to retreat.

Birhakim was much more fortified than the Austrians had estimated, and the Serbs defending it were much more tenacious than the British, and from 17 May the Croatian 7th Division made no progress in its assault. Now that they had cleared a few passages in the minefield, Stett was preparing to launch an all-out attack.

However, the Serbs suddenly retreated. Stankovic led his remaining 9,000 men to fight their way out of the siege during the night and retreat to Serum.

The next morning, when the Austro-Hungarian army stormed Birhakim, all that was left here was more than 1,000 wounded and discarded heavy equipment left by the Serbs.

The stubborn defense of the Serbs at Birhakim bought the British valuable time for the retreat from Tobruk. On 25 May, the Marines finally sent a regiment to Serum, but they could only watch as the retreating British army rolled past them and retreated towards Egypt, but they were powerless to stop it.

Hovald entered Tobruk, but the purpose of the campaign was not achieved, and the British withdrew to Egypt almost intact. The talk is that the aviation units are constantly throwing bombs over their heads, and they have been creating some problems for the retreat of the British. General Weems ordered all the warships and transports to be blown up and sunk in the harbor, and then the troops retreated from land, and black smoke was billowing from the harbor of Tobruk, and the harbor was littered with wreckage of the sunken ships.

The fall of Tobruk and Sellum shocked London, and there was much reason to believe that the Austrians were preparing a major attack on Egypt and were preparing to occupy the entire Suez Canal zone in one fell swoop.

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