Tobruk

In September 1942, Rommel's Africa Korps is only 90 miles (144Â km) from the Suez Canal, but running dangerously low on fuel. The British approve a plan to destroy German fuel bunkers at Tobruk in an attempt to cripple Rommel's attack.
The author of the plan, Canadian born Major Donald Craig (Rock Hudson) had been captured by Vichy French forces. As his expertise is considered essential to the success of the raid, Craig is rescued by Captain Kurt Bergman (George Peppard) of the Special Identification Group (SIG) and some of his men, German Jews serving with the British. They then join up with commandos of the Long Range Desert Group (LRDG), under the command of Lieutenant-Colonel John Harker (Nigel Green), at Kufra in south-eastern Libya.
Colonel Harker explains they have eight days to get to Tobruk and destroy the fuel depot and German field guns protecting the harbour, before a scheduled amphibious landing and a bombing raid on the city by the RAF. They are to drive there through enemy territory posing as POWs escorted by the SIG pretending to be guards. Once they reach Tobruk they would then link up with a full Naval and RAF assault on the city and their primary objective, Rommel's underground fuel bunkers. Craig is highly skeptical of the operation, claiming that "Staff has a genius for sitting on its brains and coming up with perfect hindsight", stating that when he originally submitted the plan they could have blown up the fuel bunkers with only a handful of men before questioning how they were supposed to get through well established German defences now. While warning Craig not to let personal differences of opinion interfere with the operation, Colonel Harker also reveals that he was the genius with perfect hindsight who convinced Staff to approve Craig's original plan which would now be "maximum effort, land, sea and air".
On the way, they encounter a patrol of Italian tanks, which stops a short distance from where they are resting in a gully. Later that night, Sergeant Major Jack Tyne (Jack Watson) spots an approaching tank column. Colonel Harker, surmising that "They must be German. The Italians are too fond of comfort to travel this late", tricks the two units into attacking each other by firing mortars, first at the Germans and then the Italians, enabling the raiders to sneak away.
To avoid detection the next day, Craig safely guides them through a German mine field, before they are attacked by a British Curtiss P-40 Warhawk fighter. They manage to shoot it down, but 8 men are killed and both of their radios are destroyed. The fighting attracts Tuareg tribesmen, who are friendly with the Germans. Craig, who speaks their language, exchanges some guns and ammunition for two prisoners.
The prisoners turn out to be British traitors Henry Portman (Liam Redmond), who has an Irish accent, and his daughter Cheryl (Heidy Hunt), who were shot down while flying from Benghazi to Cairo. They have papers signed by the Grand Mufti of Jerusalem (Mohammad Amin al-Husayni) and German Field Marshal Albert Kesselring: an agreement for "a group of important" Egyptian army officers to rise up against the British in a "holy war" with Cheryl Portman claiming that the Turks could put 4 million men into the war against Russia.
That night, the Portmans are told by a mysterious SIG member about the British masquerade and where to find a gun and a map to an underground telephone cable nearby where they can contact German command in Tobruk and alert them to the British column and upcoming attack. When they reach the telephone, however, they are spotted by an Italian patrol. Henry Portman fires at the patrol and is killed, while his daughter is seriously wounded.
Harker sends Bergman and Sergeant Krug (Leo Gordon) after the Portman's where they retrieve Cheryl from the Italians. When Krug asks how they knew about the telephone, Bergman replies, "Very simple. One among us is the enemy". Harker has Bergman and his men disarmed and then gives Bergman two hours to identify his traitor. The traitor kills Cheryl Portman so she cannot reveal his identity. Lieutenant Max Mohnfeld (Guy Stockwell), Bergman's second in command, appears from the tunnel the Portmans used to escape. He states the traitor is down the shaft. They find Corporal Bruckner (Robert Wolders), one of Bergman's closest friends for 10 years, stabbed to death. Cheryl Portman had died from cyanide, and Bruckner's suicide tablet is missing. Bergman, however, is not convinced his friend was the traitor.
The group passes through check points just outside Tobruk and after travelling through the city, they discover to their surprise that Rommel has amassed his total reserve strength at Tobruk undetected: two full panzer divisions. The discovery of Rommel's tanks puts the planned Naval assault in jeopardy, though without their radios the group have no way of warning Staff without taking the German transmitter located in the city which would alert the Germans.
The RAF then bomb Tobruk as scheduled. The LRDG blow up two of the harbour guns, and Harker orders Sergeant Major Tyne to signal the ships to abort the landing before the German tanks, which have pinned down Harker's troops, can "cut them to pieces". Harker also orders Lieutenant Boyden (Anthony Ashdown) to capture the Jerry transmitter in the city in order to cancel the landing and inform Staff of the Kesselring document. However, Boyden is killed during the bombing raid, as are Privates Alfie (Norman Rossington) and Dolan (Percy Herbert) when they discover millions in English pound notes the Germans had taken after capturing Tobruk from the British and attempt to steal the money, while Mohnfeld, who volunteered to join them as Lt. Boyden did not speak German, is knocked out.
Bergman and three of his men escape on sidecar motorcycles and manage to destroy a tank and use flame throwers to buy Harker time, however Bergman and his men are eventually killed. Meanwhile, Craig, Krug, and two other SIG men use the distraction to escape and seize a German tank well inland. After they use the tank to destroy the fuel depot, Harker and his men surrender.
After surrendering, Mohnfeld then appears where he reveals that he is really a German intelligence officer named Von Kruger, explaining that he had told the truth the night before, only altering that "The Jew found me in the tunnel" and asks Harker for the Kesserling document. However, upon seeing the destruction of the fuel bunkers, Harker had burned the paper knowing its importance to the Germans. Harker then kills Von Kruger with his pistol and is himself shot dead.
Craig, Krug and the two others manage to escape and exhausted after traveling over 70 miles on foot, make it to a scheduled back up rendezvous with a Royal Navy ship at Sallum just over the Egyptian border.

Question: What is their primary objective?
Answer: Rommel's underground fuel bunkers.