They were designed before airplanes became important in war, and (except for Fort Drum) were vulnerable to air and high-angle artillery attack, being protected only by camouflage.

[1] In an exercise in 1907 at Subic Bay, a US Marine battalion of the Advanced Base Force commanded by Major Eli K. Cole emplaced forty-four heavy guns for coast defense in a ten-week period, due to the Eight-eight fleet war scare with Japan.

[12] The last new armament in HD Manila Bay until 1940 was significant but small in quantity: Batteries Smith and Hearn at Fort Mills, completed in 1921.

One result of the Washington Naval Treaty was the diversion of twelve 240 mm howitzers on a ship bound for the Philippines to Hawaii, where they were placed on fixed mountings on Oahu.

When the ship was spotted, some accounts state that Colonel Paul Bunker, commander of the Seaward Defenses, ordered that the minefield remain active.

[21] Accounts state that US Army officers informally told Filipino reporters that the mines were placed in safe mode immediately after the sinking.

[20][22][23][24] The main part of the Malinta Tunnel complex was built on Corregidor from 1932 to 1934, with construction continuing until the Philippines was invaded by Japan in December 1941.

Due to the Washington Naval Treaty's prohibition on new fortifications, most of the complex was built without appropriated funds, using Filipino convict labor for unskilled tasks, and explosives slated for disposal.

During the siege, the Malinta Tunnel proved important to the survival of the Philippine government, the military high command, the medical staff, and numerous civilians.

[36] This situation had been anticipated in the prewar War Plan Orange-3, under which the forces in the Philippines were expected to hold out at the mouth of Manila Bay for six months.

General MacArthur had hoped to defend the Philippines more aggressively under the Rainbow Plan, and was able to get some reinforcements in the months prior to the U.S. entering the war, but this fell apart with the rapid Japanese advance in December 1941.

With almost all of the US Pacific Fleet's battleships sunk or damaged at Pearl Harbor, and the Japanese advancing in several parts of Southeast Asia at a much greater rate than expected, no relief was organized.

Although extensive guerrilla operations were conducted by Filipinos with US support, US forces did not return to the Philippines in strength until the invasion of Leyte Gulf in October 1944.

Along with mail and important documents, Trout was loaded with 20 tons of gold and silver previously removed from banks in the Philippines before departing.

[38] One aspect of MacArthur's Rainbow Plan was the Inland Seas Project, intended to defend a shipping route to keep his forces supplied.

[39][40] Although the US and Filipino forces achieved success in defending Bataan through the end of February,[41] they had taken 50 percent casualties and were worn out and poorly supplied.

[42] Also, the British fortress of Singapore had surrendered on 15 February, and the Japanese had taken several major islands of the Dutch East Indies, essentially preventing any reinforcement of the Philippines.

It was not until 20 March that the extent of Wainwright's authority and degree of independence from MacArthur was clarified by a message from General George C. Marshall, the Army chief of staff.

[44] The Japanese in Bataan received substantial reinforcements and replacements in March, including 240 mm howitzers and aircraft, and prepared for an offensive scheduled for 3 April.

[45] It started with a five-hour air and artillery bombardment that destroyed many of the Allied defensive positions and stunned the defenders; a three-day assault threw them back along much of the line.

[46] On 6 April the US and Filipino forces attempted a counterattack, which ran into a fresh Japanese attack that eventually threw the Allies further back.

However, after an initial response from a 155 mm GPF battery, Lt. Gen. Wainwright prohibited counter-battery fire for three days, fearing there were wounded POWs on Bataan who might be killed.

[51] Forts Frank and Drum had been bombarded from the Pico de Loro hills on the Cavite province shore since 6 February by a gradually increasing Japanese artillery force.

[52] The bombardment by high-angle artillery and aircraft gradually destroyed the utility of almost all of Corregidor's big guns, which had no overhead protection except for magazines and generators.

General Wainwright felt certain that further Japanese troops would land in the night and seize the Malinta Tunnel, where they might massacre the wounded and noncombatants.

[57] The units in the south were in much better positions for both supplies and continued resistance than those at Bataan or Corregidor were, and their commanders believed Wainwright's surrender orders were made under duress.

Some units never did surrender, and became nuclei for guerrilla operations that continued until the Japanese were mostly killed or captured in early 1945, following MacArthur's return to the Philippines in force in October 1944.

By the final surrender on 9 June, the Battle of Midway was over, blunting Japan's naval strength with the loss of four large aircraft carriers and hundreds of skilled pilots.

Both of these victories were costly to the US Navy as well, with two aircraft carriers lost, but the United States could replace their ships and train more pilots, and Japan, for the most part, could not do so adequately.

Rock Force cleared the bunkers in the typical fashion of the war in the Pacific: air-delivered napalm bombs where needed, followed by assaults with flamethrowers and white phosphorus grenades among other weapons.