'Topside' was the US semi-official designation of the airborne operation to parachute 2,050 men of Lieutenant Colonel George M. Jones’s 503^rd Parachute Infantry, 462^nd Parachute Field Artillery Battalion and 161^st Airborne Engineer Battalion onto Corregidor island’s western end, known as Topside, in the mouth of Manila Bay in the Japanese-occupied Philippine islands (16 February 1945).

One of the most important fortresses in the South-West Pacific as it commands the entrance to Manila Bay, the island is shaped like a tadpole with a length of 3.5 miles (5.6 km) and a width of 1.5 miles (2.4 km) across its head. The isthmus connecting the head and tail was known as Bottomside, and to the east of this is Malinta Hill, which in 1941 contained the headquarters of the island’s US garrison, in a tunnel dug deep into the hillside. The eastern side of the head was known as Middleside and accommodated the officers' living areas while the western side was Topside, rising 500 ft (150 m) above the sea, was occupied by the enlisted men’s barracks and parade grounds. The island was covered with trees and scrub, but possessed a good road network and Kindley Field, with a 2,400-ft (730-m) runway, was located on the flat eastern end of the tail.

Corregidor was extremely well defended with fixed defences including powerful coastal artillery in the form of 23 batteries of 56 pieces of artillery ranging in calibre from from 3 in (76.2 mm) to 12 in (305 mm).

Fort Hughes, built on the nearby island of Caballo, consisted of three hills reaching a maximum height of 380 ft (116 m) and had 11 batteries of 16 pieces of artillery in calibres up to 14 in (356 mm). Fort Drum, built on an islet slightly more than 4 miles (6.4 km) to the south-south-east of Corregidor, had been built up into a concrete behemoth whose battery comprised two twin 14-in (356-mm) gun turrets and four 6-in (152-mm) guns. Fort Frank, on Carabao Island 7.5 miles (12 km) to the south of Corregidor and just 1,500 ft (455 m) from the southern shore of Manila Bay, had 19 guns of up to 14-in (356-mm) calibre.

This defensive complex was manned by 5,700 US and Philippine Scouts troops of the 59^th, 60^th, 91^st and 92^nd Coast Artillery Regiments, and its capabilities made Manila Bay and its magnificent harbour all but impenetrable to a conventional attack from the sea. As long as the defences held, even if Manila itself fell, the harbour would be useless to the Japanese. However, the largest anti-aircraft guns were of only 3-in (76.2-mm) calibre and unable to reach high-flying Japanese bombers. Corregidor was also vulnerable to artillery barrage from the tip of the nearby Bataan peninsula, only 2 miles (3.2 km) to the north, and could not be hold for any sustained period should Bataan fell.

During the first Philippine campaign, following the Japanese 'M' (ii) invasion of the Philippine islands group, the US and Filipino forces were compelled to retreat into the Bataan peninsula to make a last-ditch stand against the Japanese. General Douglas MacArthur, the commander-in-chief, shifted his headquarters onto Corregidor and placed Major General Jonatham M. Wainright, commander of the North Luzon Force, in command on Bataan. Corregidor was defended by the 4^th Marines, a unit which had been evacuated to the Philippine islands from China shortly before the outbreak of the Pacific War. When MacArthur was ordered on 11 March 1942 to escape to Australia, Wainright was promoted to lieutenant general on 19 March and took command on Corregidor, leaving Major General Edward P. King, his successor in command of the North Luzon Force, in immediate command on Bataan.

King’s line collapsed on 6 April, and King felt that he had no alternative but to surrender what was left of command two days later. The Japanese immediately began moving heavy artillery to the southern tip of Bataan peninsula and began a systematic reduction of the artillery emplacements on Corregidor. The first Japanese battalion, of the 61^st Regiment, landed on Corregidor during 5 May at the tail of the island near the airstrip and encountered fierce resistance from the marines. The Japanese managed to bring a second battalion and several light tanks ashore, however, and the marines lack adequate anti-tank weapons with which to meet this greater threat. Wainright surrendered Corregidor on the following day as the Japanese brought in a third battalion and were closing on the Malinta Tunnel with its underground hospitals. The overall US casualties were some 800 killed and 1,000 wounded. Japanese casualties were very heavy but the exact number is uncertain. Claims have been made that there were in the order 900 dead and 3,000 wounded, which would be the better part of the three attacking battalions.

An aerial bombardment to soften Corregidor’s defences started on 13 January, and daily attacks by heavy bombers continued until 16 February.

By February 1945 the Japanese garrison on Corregidor numbered 6,000 men under the command of Colonel Itagaki Akira, and this garrison had constructed additional field works. MacArthur’s intelligence branch estimated the garrisons of Corregidor as just 850 men and of Bataan as 6,000, however, and MacArthur assigned the 151^st Regimental Combat Team, under the command of Brigadier General William C. Chase, commander of the 38^th Division, to take Mariveles on 15 March 1945. The landing force was to embark at Subic Bay in vessels of Rear Admiral Arthur S. Struble’s 9^th Amphibian Group, and Corregidor was to be assaulted on 15 February by an amphibious force drawn from the 34^th Infantry and a parachute landing by part of the 503^rd Parachute Infantry. Minesweepers were to start clearing the approaches on 13 February and be followed by a bombardment force under Rear Admiral Russell S. Berkey, the commander Cruisers, 7^th Fleet.

As well as arriving at a gross underestimation of the Corregidor garrison, MacArthur’s intelligence also greatly overestimated the Japanese strength on Bataan, which was a mere 1,400 men, very few of them anywhere near Mariveles. Berkey was highly worried by the lack of Japanese reaction to the minesweeping effort, which he had hoped would persuade the Japanese into revealing their artillery positions. Berkey’s ships began their gunfire bombardment at 09.43 on 14 February and the Japanese finally responded by firing on the minesweepers from Corregidor and Caballo. The fire was intense until 10.18, when the minesweepers completed the clearance of 76 mines and withdrew without loss.

The destroyer force off Mariveles had a harder time of it. The destroyer Fletcher was struck by a 150-mm (5.91-in) shell hit which started a brief fire, the yard minesweeper YMS-48 was set on fire, and the destroyer Hopewell took four hits which caused 19 casualties and compelled the ship’s withdrawal to Manus island for repair. The minesweepers then moved into the harbour, but the destroyers La Vallette and Radford both struck mines and were forced to withdraw. The destroyers were in waters that had already been swept twice, suggesting the Japanese had used more sophisticated mines than their usual moored contact mines.

The landing force arrived at 10.00 on 15 February and encountered negligible opposition ashore, but the medium landing ship LSM-169 hit yet another mine. At a time early in the morning of the following day, about 20 kamikaze boats penetrated into the harbour of Mariveles and sank the landing craft LCS-7 and LCS-49, and disabled LCS-27.

The area best suited for an airborne assault on Corregidor was Kindley Field, but this would require the troops then to fight uphill against strong fixed defences, so the decision was made to run the tactical risk of dropping the force on the small parade ground and golf course on Topside. The preliminary bombardment by eight cruisers and 14 destroyers began at 06.30, and was followed by an air attack at 08.00 by 36 Consolidated B-24 Liberator heavy bombers and 31 Douglas A-20 light attack bombers. From 08.00 on 16 February, attacks by 36 Consolidated B-24 Liberator heavy bombers and 60 minutes of low-altitude bombing and strafing runs by Douglas A-20 light attack bombers preceded the airborne and seaborne landings. Although these preliminary bombardments inflicted only comparatively light casualties on the Japanese in their deep defences, it stripped most of the vegetation off the island, and also triggered landslides which sealed some 2,000 of the defenders inside the Malinta Tunnel.

The airborne forces co-operated with sea-landed elements of Colonel Aubrey S. Newman’s 34^th Infantry within Major General Roscoe B.Woodruff’s 24^th Division in the reduction of the island. At the same time that the paratroopers landed on Topside, the first wave of Lieutenant Colonel Edward M. Postlethwait’s 3/34^th Infantry had waded ashore and established a beach-head at San Jose Point on the eastern end of Corregidor. The succeeding waves took the brunt of the hastily organised Japanese defence, and several landing craft and infantrymen fell victim to mines. The battalion pushed inland against sporadic resistance, mostly from groups coming out of the subterranean passages of the inland to ambush the advancing US troops. Two of the 3/34^th Infantry’s companies managed to fight their way forward to take the road and both the northern and southern entrances to Malinta Hill, Company A stationed itself near the waterline. Company I landed at North Dock and occupied the harbour. The infantrymen intended to keep the Japanese troops inside the tunnel as other units moved inland, accompanied by tanks and flamethrower units that devastated pillboxes and tunnels in the surrounding areas held by the Japanese. For eight days until 23 February, these units staved off successive banzai charges, mortar attacks, and even a suicide squad with explosives strapped to their bodies, killing more than 300 Japanese.

The first paratroopers top land were men of the 3/503 Parachute Infantry, who began their jump at 08.30. Most came down on their designated dropping zones, but a few had to be rescued from the harbour by PT-boats. By chance, one group of 25 paratroopers came down on top of Itagaki’s position, surprising and killing the Japanese commander. This destroyed the cohesion of the Japanese defence almost from the start of the battle. The paratroopers also suffered heavily, with 25% casualties from landing mishaps and Japanese fire. However, the paratroopers quickly secured positions overlooking the planned amphibious landing beaches at Bottomside. At 10.30 the assault battalion of the 34^th Infantry came ashore in five waves in the area to the south of Malinta Hill, and only the fifth wave took any significant fire. A second wave of paratroopers dropped at 12.30, landing precisely on target. However, casualties were so high from Japanese fire at the descending paratroops that a third wave, scheduled to jump on the following day, was instead delivered by landing craft.

The defeat of the dug-in Japanese took 10 days characterised by slow US advances during the days and bloody but futile Japanese banzai charges during the nights. Almost the whole Japanese garrison was killed.

The most ferocious part of the battle to regain Corregidor occurred at Wheeler Point on the night of 18/19 February and involved Companies D and F of the 2/503^rd Parachute Infantry. Settled in defensive positions near Battery Hearn and Cheney Trail, at 22.30 under a black, moonless sky, the men of the two companies came under attack by 500 Japanese naval infantrymen, who emerged from the Battery Smith armoury and made a banzai charge. Company F checked the attack of the Japanese trying to break through to the south. Except for flares fired throughout the night by warships lying offshore, the three-hour battle was decided only with rifles, automatic weapons and the courage of the 50 paratroopers ranged against the Japanese. Not all men of the company were involved in the fighting because of the ensuing confusion. The encounter ended in Japanese defeat, with more than 250 Japanese corpses strewn along a 200-yard (180-m) stretch of Cheney Trail; Company F suffered 14 dead and 15 wounded. This was the last attack of any significance by the Japanese on Corregidor.

The Japanese trapped in the Malinta Tunnel tried to blast their way out with explosives on the night of 20/21 February, but the detonation of these explosives triggered an explosion in a major magazine, whose detonation annihilated most of the Japanese and buried six paratroopers in landslides outside the tunnel. After the explosions and rock falls had ended, some 50 Japanese exited the cave to attack and the Americans shot them down. Two nights later there was a similar attack. On 24 February the US troops start to clear Corregidor’s tail, advancing behind two tanks brought in with the amphibious landings. The island was rocker by as second huge explosion on 26 February, when the Japanese holding out in the Radio Tunnel, which had been constructed in 1939 to shelter Corregidor’s signals intelligence unit, detonated their own magazine. The explosion turned one M4 Sherman medium tank over, killed 52 Americans and wounded another 144, and killed the 150 Japanese in the tunnel. The next day the last organised resistance on the island collapsed.

Only isolated pockets of resistance continued to fight on with a suicidal determination until 26 February, when Corregidor was finally declared secure. Large numbers of Japanese drowned while attempting to swim to the mainland. Some thousands more sealed themselves in the island’s many tunnels and opted to commit suicide rather than surrender. Thus only a very small number of Japanese were captured. By 1 March Manila Bay was open for Allied shipping. The Battle of Corregidor had cost the 503^rd Parachute Infantry 169 killed and 531 wounded, and the 34^th Infantry 38 killed and 153 wounded. Of the 2,065 men of both lifts by the 503^rd Parachute Regimental Combat Team, about 280 had been killed or severely injured.

The overall US casualties were 455 men killed and 550 wounded, which was nearly a third of the attacking force. More than 4,200 Japanese dead were counted, only 19 men surrendered, and an estimated 200 were killed while trying to swim away and 500 were buried alive in the Malinta explosion. Some 20 Japanese stragglers emerged from concealment on 1 January 1946, more than four months after the end of the war.

The other Japanese units defending Manila Bay were also destroyed, starting with Ternate on the south of the bay on 2 March and Fort Hughes on 27 March. The latter was assaulted by a battalion combat team of the 151^st Infantry, which overran the island the first day but had then to spent four days in trying to prise the Japanese out of out their tunnels. Engineers then tried pouring Diesel oil into the tunnels and igniting it, but this also failed until the US Navy supplied a pipeline, pump and two oil barges to allow 2,500 US gal (9465 litres) of oil to be pumped into the tunnels and ignited with white phosphorus. This treatment was repeated over the next two days, and the last Japanese survivor was killed by a patrol entering a tunnel on 13 April.

Fort Drum was taken in the same way. The troops landed away from the entrance, which commanded the approaches with potent cross-fires, and pumped 3,000 US gal (11355 litres) of oil into the fortress from its roof. This was followed by a 600-lb (272-kg) explosive charge on a delayed timer. The troops then pulled back and the charge went off, setting off secondary explosions which continued for days and completely wrecked the fortress.

Fort Frank was assaulted on 16 April and found to have been evacuated.