Although specifically prohibited by the Treaty of Versailles, the Japanese Navy began development of defensive military sites in Palau in 1933 and by the start of World War II, Palau was an important forward naval supply base. Early in the war, Japanese units flew sorties from Palau's large deep-water anchorages for the attack on Darwin, Australia, as well as for the re-supply of troops and supplies in New Britain and New Guinea. By the spring of 1944, large contingencies of Japanese Imperial Army and Navy were entrenched throughout Palau under Lt. Gen. Inoue Sadae, the recently appointed commander of the Palau Sector Group.
Recognizing that Palau would be a potential target of Allied forces on their way to Japan, frantic last-minute efforts by the Japanese to fortify this chain of islands resulted in substantial improvements of defensive positions, particularly around its two key airfields, one on Babeldaob, Palau's largest island, and one on Peleliu in the south; this included adding large numbers of fighters and bombers. Just south of Babeldaob lies Koror which was, at that time, the headquarters for all Japanese forces in that part of the Pacific. The entire island was heavily protected by a ring of well placed anti-aircraft (AA) batteries.
Although the Japanese military did not then know it, Palau was never considered a prime strategic target on the road to Tokyo. But during a span of six short months in 1944, it got in the way of Doug MacArthur's plans on two separate campaigns. For both, Chester Nimitz came to the rescue. The invasion of Hollandia was scheduled for mid-April 1944.
A glance at the map quickly reveals that Japanese aircraft from Palau were well within range of northern New Guinea and could be a flanking threat to MacArthur's attack there. Nimitz's Central Pacific Advance, having taken the Marshall Islands ahead of schedule and getting ready for his June invasion of the Mariana Islands northeast of Palau, was now in a position to deal with this problem for MacArthur. On March 23, three aircraft carrier groups from Task Force 58 flew sorties from Majuro Atoll in the Marshall Islands and headed west under the code name Operation DESECRATE ONE. On March 30 and 31, U. S. Navy aviators attacked Palau with three general objectives: destroy enemy aircraft, sink naval and merchant shipping and lay aerial mines to prevent escape of any remaining enemy shipping from follow-on attacks.
During the two-day strike, over 160 enemy aircraft were destroyed, primarily on Peleliu, at a cost of 7 U. S. aircraft lost in combat. During the same two-day period, the U. S. forces claimed 29 Japanese ships sunk with an additional 18 damaged. Aerial mines were seeded in key channels to prevent escape of any surviving vessels. DESECRATE ONE was a success. The results, combined with the subsequent U. S. successful invasion of Hollandia, abruptly made Palau useless as a Japanese naval base; it ceased as a military anchorage for the remainder of the war. But 35,000 Japanese Army and Navy personal were now trapped throughout the well-fortified Palaus with no means of escape or surface re-supply. But two airfields could still receive aircraft from the Marianas and the Philippines.