Naval Base Ulithi was a major United States Navy base at the Ulithi Atoll in the Caroline Islands in the western Pacific Ocean, to the north of New Guinea during World War II.

The base was built to support the island-hopping Pacific War efforts of the Allied nations fighting the Empire of Japan.

At the start of World War I in 1914 British warships destroyed the German colonial plantation owner's radio station.

The United States Army Air Forces bombed the Truk base, but it was bypassed in the amphibious landing war.

The survey ship USS Sumner found the lagoon was well protected and usable for fleet anchorage, with depths ranging from 80 to 100 feet.

The main projects were building a large fleet recreation center on Mogmog Island for 20,000 troops, docking piers, small plane airstrips, seaplane base, pontoon piers and camps.

Most supplies like fuel, ammunition, and spare parts were stored in cargo ships in the atoll and unloaded as needed.

On 10 October 1944, part of the 6th Special Battalion arrived at Ulithi for unloading and loading of ships, the 6th handled over 12,000 tons of cargo per month.

[14][15][16] The United States Fifth Fleet used Ulithi for staging, including: Philippines campaign, Operation Hailstone and the troop landings at Okinawa.

In March 1945 106 destroyers, 29 aircraft carriers, 15 battleships, and 23 cruisers departed for Okinawa.

[19][20] In October 1944 a carrier battle group from Task Force 38 of the United States Third Fleet under William Halsey arrived at Ulithi for resupply.

A court of inquiry was held aboard the destroyer tender USS Cascade on 26 December to determine if Halsey should be sanctioned for sailing into the typhoon.

The court found "errors in judgment committed under stress of war operations and stemming from a commendable desire to meet military requirements".

[22][23] The fleet oiler USS Mississinewa (AO-59) was sunk on 20 November 1944 at Ulithi after being hit by a Japanese kaiten-manned torpedo.

Over 100,000 barrels of fuel oil were stored at Ulithi in tanker ships and land depots.

[24] On 11 March 1945, kamikaze aircraft hit USS Randolph (CV-15) on her stern starboard side in a nighttime raid on Ulithi.

[35][36] US Navy Seabees turned the swamps and forest on Mogmog Island (Mog Mof) into a large Ulithi fleet recreation center.

At the depot Seabees stored all the supplies and gear needed to build and maintain the base at Ulithi.

Mogmog Airfield was constructed for light aircraft for short trips to the other nearby islands.

[37][38] Naval Base Ulithi headquarters and the military cemetery were built on Asor Island.

The submarines attacked warships and sank supply ships that were needed by Japan to resupply their many bases in the Pacific.

US subs also did reconnaissance patrols, landed guerrilla special forces and search and rescue missions for downed aircrew men.

[44] US submarines had long ranges, but needed to be resupplied with fuel, food, torpedoes and deck gun shells.

While the submarine was being resupplied, and repaired if needed, crews could have a break (R&R) at the Ulithi's fleet recreation center on Mogmog Island.

The Navy had built special auxiliary floating drydocks that were able to repair battle damage to even the largest ships and do regular maintenance in the field saving ships trans-pacific travel time for repair.

The USS Bennington (CV-20) was repaired with a badly damaged flight deck from Typhoon Cobra.

Seaplanes did reconnaissance patrols and search, also rescue missions for downed aircrew mem and survivors of sunk ships.

The most common seaplanes at the base were Consolidated PBY Catalina and Martin PBM Mariner.

Seaplane tenders and land base had stores to supply: food, fuel, ammo, spare parts.

Destroyers were used to protect capital ships like aircraft carriers, battleships and heavy cruisers.