This was the US bombing of the Ploieşti oilfields in Romania (1 August 1943).

These oilfields and their eight associated refineries produced about 10 million tons of fuel annually, a tonnage which was of vital importance to the German war effort. For this reason, therefore, Ploieşti was designated a major strategic target for Allied bombing as early as January 1942 after the first attack had been launched on 23 June 1941 by Soviet bombers, and the first US attack was mounted on 11 June 1942 by the 12 Consolidated B-24D Liberator heavy bombers of Colonel Harry A. Halverson’s HALPRO Detachment in Egypt. The damage caused in this initial pair of raids was minimal, but the US attack served to confirm to the Germans the importance and the vulnerability of the target area.

Thus rapid steps were taken to improve Ploieşti’s defences (fighters, Flak and smoke generators) so that other raids would face a more difficult task. General Alfred Gerstenberg, the overall commander of air defences for the Axis forces in Romania from 15 February 1942, had used the HALPRO Detachment’s attack as justification to build one of the heaviest and most fully integrated air defence networks in Europe, with some 25,000 men in Ploieşti and 11,000 near Bucharest. Defending the Ploieşti area specifically, Gerstenberg had several hundreds of 88- and 105-mm (3.465- and 4.13-in) anti-aircraft guns, as well as considerably larger numbers of smaller-calibre weapons camouflaged in hay stacks and false structures. A number of pieces were also ingeniously disguised on false-sided rolling stock to provide mobile air defences along the surrounding railway. Gerstenberg tied these defences into a radar network capable of controlling 52 Messerschmitt Bf 109 single-engined and Messerschmitt Bf 110 twin-engined fighters along with a number of largely obsolescent Romanian and wholly obsolete Bulgarian fighters against any attackers. Gerstenberg also relied on signals intelligence based in Athens, Greece, to achieve greater awareness of preparations being made by Lieutenant General Louis H. Brereton’s US 9^th AAF from as far away as North Africa.

The major USAAF effort was planned as 'Statesman' and 'Soapsuds', but finally came to fruition as 'Tidal Wave' under the overall control of Major General Carl A. Spaatz’s North-West African Air Force and tactical command of Brigadier General Uzal G. Ent. Five groups of B-24 bombers were allocated to the mission, namely the North-West African Air Force’s own 98^th and 376^th Bombardment Groups (Heavy) led by Colonel John R. Kane and Colonel Keith R. Compton respectively, from the UK Major General Frederick L. Anderson’s US 8^th AAF in the form of the 44^th and 93^rd Bombardment Groups (Heavy) led by Colonel Leon W. Johnson and Lieutenant Colonel Addison E. Baker respectively, and Colonel Jack W. Wood’s 389^th Bombardment Group (Heavy) initially allocated to the 8^th AAF but diverted while still in the USA.

US tactical planning under the supervision of Colonel Jacob E. Smart called for the mission to be despatched from Benghazi on a low-level attack in the hope of taking the Germans and Romanians by tactical surprise. The plan was based on the experience of the HALPRO raid. This had encountered only minimal air defence, so the 'Tidal Wave' plan was conceived as a daylight undertaking in which the bombers would approach at low altitude to avoid detection by German radar. The mission training included extensive use of detailed sand table models, practice raids over a mock-up of the target in the Libyan desert, and tactical exercises over a number of secondary targets during July to prove the viability of this type of low-altitude attack.

The bombers to be used were revised with bomb-bay fuel tanks to increase their capacity to 3,100 US gal (11735 litres). The operation was to to be flown by 178 bombers with 1,751 aircrew, and was one of the largest US heavy bomber commitments up to that time. The aircraft were to take-off from airfields near Benghazi in Libya, cross the Mediterranean and Adriatic Seas, pass near the island of Corfu, cross over the Pindus mountains in Albania, traverse southern Yugoslavia, enter south-western Romania, and finally turn east toward Ploieşti. Approaching Ploieşti, the bomber crews were to locate predetermined checkpoints, fly toward their targets from the north, and bomb all the targets simultaneously.

Some 178 Liberator bombers took off on 'Tidal Wave', but one of these was lost almost immediately after lifting off. The other 177 aircraft continued, and over the Mediterranean separated into five streams for the approach flight, which was already being monitored by the alert Germans. Another Liberator went out of control over the Adriatic Sea and crashed, another could not regain the necessary altitude after descending to look for survivors, and 10 more aircraft turned back to friendly bases when a measure of confusion which now developed was compounded by the difficulty of retaining unit cohesion as a result of the strict radio silence, which was maintained in accord with the mission orders.

The remaining 165 Liberator bombers now had to climb to pass over the 8,700-ft (2650-m) height of the Pindos mountains, which were shrouded in cloud cover. Although all five groups made the climb to about 11,000 ft (3350 m), the 376^th and 93^rd BG(H)s, using higher power settings, began to move ahead of the other three groups, resulting in variations in speed and time which disrupted the synchronisation of the group attacks that had been deemed so important by Smart. This threat to successful execution of the attack was thought by the senior leadership to be of secondary importance to the mission’s operational security. Although the need to rebuild the formations was clear, and would have demanded the breaking of radio silence, the attack in fact continued without correction, an error of judgement which was to prove costly.

Now well strung out on their approach to Piteşti, all five groups nonetheless reached the navigational checkpoint 65 miles (105 km) from Ploieşti At Câmpina, and the 389^th BG(H) departed as planned for its separate but synchronous approach to the mission target. Continuing from Piteşti, Compton and Ent made a navigational judgment that was to prove very costly: at Târgovişte, mid-way to the next checkpoint at Floreşti, Compton followed the incorrect railway line for his turn toward Ploieşti, setting his group and Baker’s 93^rd BG(H) on a course for Bucharest. In the process, Ent and Compton overrode the advice of their bomber’s navigator, Captain Harold Wicklund, who was a HALPRO veteran. In the face of an impending disaster, many crews chose to break radio silence and draw attention to the navigational error as both the groups flew straight into Gerstenberg’s air defence complex around the Bucharest area, which they now had to now face before reaching that still awaiting them around Ploieşti. Pressing on through a storm of Flak, Baker’s bomber suffered damage so severe that Baker had to order the bombs to be jettisoned in order that his aeroplane could continue to lead the formation over its target at the Columbia Aquila refinery.

Despite his group’s heavy losses, Baker continued the the 93^rd BG(H)'s course and, once clear, began to climb. But appreciating that the aeroplane was effectively lost, Baker maintained the climb in order to permit the crew to abandon the bomber: none of the men survived. The two aircraft flown by Major Ramsay D. Potts and Major George S. Brown flew into heavy smoke over Columbia Aquila, but led two additional elements of the 93^rd BG(H) to make a successful drop of their bombs over the Astra Romana, Unirea Orion, and Columbia Aquila refineries. The 93^rd BG(H) lost 11 aircraft over its Ploieşti targets. One of the losses was a bomber despatched by a Romanian IAR-80 fighter, which half-rolled and passed inverted below the bomber to rake the B-24’s belly with fire: the bomber crashed into Ploieşti women’s prison, a three-storey building which burst into flames, only 40 women surviving the disaster.

The defences were notably strong around the Romana America refinery which was 376^th BG(H)'s target, so Ent ordered Compton to attack targets of opportunity. Most of the 376^th BG(H)'s bombers attacked the Steaua Română refinery at Câmpina from the east, and five headed directly into the conflagration over the Concordia Vega refinery. At Câmpina, air-defence sites on the overlooking hills were able to fire down into the formation, and IAR-80 fighters also had some successes.

With the 93^rd and 376^th BG(H)s engaged over the target area, Kane and Johnson of the 98^th and 44^th BG(H)s made their prescribed turn at Floreşti and continued to their respective targets at the Asta Romana and Columbia Aquila refineries. Both groups found the German and Romanian defences on full alert and faced the tactical disadvantages of the oil fires which were now raging, thick smoke, secondary explosions, and delayed-action bombs dropped by Baker’s 93^rd BG(H) during its earlier run. The approach of the groups led by Kane and Johnson, parallel to the railway linking Floreşti and Ploieşti, had the unfortunate distinction of encountering Gerstenberg’s disguised Flak train. At an altitude of only about 50 ft (15 m), the bombers of the 98^th and 44^th BG(H)s found themselves to the left and right of this train’s direction of movement. The advantage lay with the 98^th and 44^th BG(H)s, whose gunners responded rapidly to the threat, disabling the locomotive and killing many of the Flak gun crews.

Despite the fact that the effects of the attacks by the 93^rd and 376^th BG(H) made it difficult to find and attack their primary targets, both Kane and Johnson did not deviate and took heavy losses. Their low approach even enabled gunners to engage in continued ground suppression of Flak gunners. Lieutenant Colonel James T. Posey took 21 of the 44^th BG(H)'s bombers on a separate assigned attack on the Creditul Minier refinery just to the south of Ploieşti. Although the Flak guns had already heavily engaged the 93^rd BG(H)'s aircraft, Posey nonetheless faced concentrated fire. Maintaining a low-altitude approach into the target area took some of the aircraft, still heavily laden, through tall grass and damage was caused by low-level obstructions. Posey and his aircraft, which were carrying 1,000-lb (454-kg) rather than 500-lb (227-kg) bombs, found their marks at Creditul Minier and took no losses.

The last element of 'Tidal Wave' bombed the Steaua Română refinery at Câmpina, some 8 miles (13 km) to the north-west of Ploieşti). The attack of the 389^th BG(H) led by Colonel Jack Wood had been carefully rehearsed at Benghazi, and the efforts of the 376^th and 389^th BG(H)s so severely damaged the refinery that it was unable to resume production for the rest of the war. The 389^th BG(H) lost four aircraft over the target area.

As the bombers made their departure from Ploieşti to the south, they flew over Bulgaria, they were intercepted by three groups of fighter groups in the form of 10 Bf 109 machines from Karlovo, four Avia B-534 machines from Bozhurishte and 10 B-534 machines from Vrashdebna (Sofia) airport. Four of the Bulgarian pilots gained the first 'kills' of the war for the Bulgarian air force.

Only 88 B-24 bombers, 55 of them with varying degrees of damage, returned to Libya. The US losses included 44 bombers downed by the air defences and a number of bombers which ditched in the Mediterranean or were interned in Turkey after making emergency landings. Other aircraft were diverted to alternate airfields such as that of one of the British bases on Cyprus. The US personnel losses were 310 men killed, 108 taken prisoner by the Axis states, and 78 interned in Turkey.

Allied assessment of the attack estimated a loss of 40% of the refining capacity at the Ploieşti refineries, but some of the refineries remained largely untouched. Most of the damage was repaired within weeks, after which the net output of fuel was greater than before the raid. Attacks on secondary targets caused casualties at Drenta, Elena, Byala, Ruse, Boychinovtsi, Veliko Tarnovo, Plovdiv, Lom and Tulovo.

In overall terms, and given the large and unbalanced loss of aircraft and the limited damage to the targets, 'Tidal Wave' was therefore a strategic failure. Ploieşti continued to exercise a fascination for the Americans, and over the following year another 20 raids were launched against the refineries and the complex’s transportation system before the refineries shut down on 24 August 1944, just six days before the Soviets reached Ploieşti. In the course of some 7,500 bomber sorties the Americans dropped about 13,500 tons of bombs on Ploieşti and lost 350 heavy bombers.