These 12 rare photos show the island city the US Navy built to invade Japan

In 1944, the U.S.'s progress in its island-hopping campaign through the Pacific brought it to Ulithi Atoll. From March to September, they bombed the Japanese forces stationed there until they eventually withdrew, believing the atoll was too small to accommodate an airfield and therefore not of value to either side.

The U.S. Navy disagreed. Forces landed in Sep. 1944 and began building one of the largest naval bases used in the war. At it's peak, Ulithi Atoll housed 617 ships, had its own 1,200-yard airstrip, and hosted 20,000 troops on its recreation island, Mogmog.

Here are 12 photos from the massive base:

1. Ulithi Atoll primarily served as a massive anchoring and refueling point for Navy ships.

2. Ulithi Atoll was home of the famous "Murderer's Row," where the Third Fleet's massive aircraft carriers were parked in late-1944.

3. Sorlen Island in Ulithi Atoll featured a 1,600-seat movie theater and a hospital. Water was pumped in from the ocean and distilled on site.

4. The airstrip was constructed on Falalop Islet. Hellcats and other planes were stationed there to protect the island and to bomb targets to the north.

5. Bombs were moved across the soft sand on trailers.

6. Mogmog Island served predominantly as a rest and recreation facility where sailors could drink, lounge, and take in entertainment.

7. An officer's club was constructed on Mogmog.

8. Religious services were held on the islands. Most of the natives consolidated onto a single island for the duration of the Navy's stay, but some visited with sailors.

9. Sailors enjoying themselves on the beach were still surrounded by their offices.

10. The Navy set up floating dry docks to maintain and repair ships at the atoll.

11. Ships at Ulithi were in danger from mines and suicide torpedo attacks. The USS Mississinewa, a tanker filled with aviation fuel, was sank in Nov. 1944 by a Kaiten suicide torpedo.

12. The suicide torpedoes were a new Japanese weapon that was analyzed at the Ulithi facilities.

Ulithi Atoll gradually drew down in size as ships moved north but remained in service through the end of the war. This video shows the sheer size of the fleet anchored there.