How did East Indies oil lose its "top billing" in the Pacific War as the war went on?

By that time, SE Asia as wild. There were a lot of inhabitants, but there were not that much infrastructures and "European" inhabitants, especially in the East Indies.

So the Japanese only had to control "useful" East Indies: parts of Java and Sumatra, Bali, Timor and that's nearly all. All those parts of East Indies were controlled, in their communications with Japan, by the Filipino archipelago.

That is why the American strategy had a debate not over conquerring or not South East Indies, but about conquerring or not Filipino archipelago (versus a straight forward assault againt the Ryu Kyu).

So to answer your question: East Indies were an important factor in the strategy:

• For the Japanese, it was about being able to keep contact between them and Japan with convoys and intermediate islands. They needed those links to use the oil of East Indies
• For the Americans, it was about cutting those links. The Allies did not need the oil resources of East Indies for their own use

Oil and strategic direction

Before we start deliberating events in WW2, let's us first briefly remind ourselves about Japanese expansion plans. In order to gain necessary raw resources and expand its sphere of influence, there were two schools of thought, or political doctrines. First one, Northern Expansion Doctrine emphasized invasion of Manchuria and Siberia, with USSR being main enemy. Second one, Southern Expansion Doctrine, emphasized expansion into Southern Asia. In this plan, main enemies would be European continental powers and possibly US.

It must be noted that Northern plan would not engage Imperial Japanese Navy (IJN) too much. Similarly, in Southern plan, Imperial Japanese Army (IJA) would have secondary functions. But it also must be noted that Northern strike would not yield much in terms of gaining oil fields. There are vast deposits in oil in Siberia, but they were discovered only after the war (1950's and 1960's) and were too far away from Japanese starting positions. Therefore, war against USSR would not solve oil crisis - Japan would still depend on imports.

Now, there are many reasons why Northern Strike Doctrine was abandoned. Invasion of China was not going as expected, Western powers did not remain neutral, in fact US started oil and trade embargo in 1939. Soviets were much harder nut to crack, in fact at Khalkhin Gol they trashed Japanese substantially. Finally, Germans in 1939 went to war against Western powers and not USSR, and even in 1941 when they invaded USSR they did not pre-warn Japanese so they were not ready for war against USSR before dreaded Siberian winter arrived.

In any case, Southern Strike Doctrine won and IJN would be main actor, but also main beneficiary. IJA was never fully mechanized, and it resembled more of WW1 army than WW2 army, especially considering European theatre standards of war. If we look at Japanese tanks of WW2, we would see that two main types (Type 95 Ha-Go and Type 97 Chi-Ha) were relatively light compared to let's say Sherman, T-34 or PzIV and were produced in modest numbers (2000 and 3000 respectively). Also, during the war in jungle they did not have prominent role due to the terrain, and many were deployed as almost static defensive boxes, or simply stored back home in Japan. In China-Burma-India theatre, for example in aforementioned Operation Ichi-Go, they were used in greater numbers, still considering vast size of Japanese Army (half-million) 800 light tanks are not that much. Similarly, number of vehicles (i.e. mostly trucks) was also limited - IJA relied mostly on horses.

Aviation (divided between IJN and IJA) was also major user of fuel, and later in the war fuel shortages curtailed training, but also reduced number of operational sorties. Note that this would also have impact on battlefield, but not equally. In operational area of IJN (SE Asia, especially New Guinea and island hopping campaign), they would face very strong aerial opponent with numerous and latest (most modern) US aircraft. This crippled Japanese operations, especially from late 1943 when fuel shortage begun to be felt. However, in China-Burma-India theatre Allied air power was much weaker, both in number of planes and their quality. Indeed, they would often used types obsolete in other theaters like for example Vultee A-31 Vengeance . Consequently, Japanese were able to move and even launch offensive operations late in the war.

What about IJN ? Carriers, battleships, cruisers ... all require lots of oil. In 1942, quantities were somewhat adequate - what Japanese did capture in Dutch East Indies replaced lost oil from imports. An then came Allied submarine campaign. Japanese merchant ship losses increased, consequently oil inventories dropped. Japanese fleet still managed to find enough fuel for major operations (or attempts of it), but this was not enough for continuous training or patrols. Especially, it was not enough for small-to-medium scale raids against vulnerable Allied targets. For example, in 1942 Japanese could mount hit-and-run strikes like bombing of Darwin against weekly defended targets. Allied fleet could not be everywhere, and it was difficult to predict movement of a cunning enemy. But later in the war, mostly due to fuel shortage and submarine threat, Japanese preferred to keep their forces in ports, and prepare for decisive battle, which they eventually lost at Philippine Sea and Leyte Gulf - not a small due to lack of training of aircrews and ship crews. To conclude, submarine campaign was so successful that direct attack on East Indies oilfields (which were heavily defended) to re-capture them was not necessary.

As for IJA, in a sense, they have lost their war even before it begun. Relegated to secondary role and theaters, they solidered on as best as they could, and confronted Allies with what they had, which was often obsolete. In the end, they were defeated simply by bypassing their defenses , with lots of units still intact in time of Japanese surender.

The reason was that the East Indies was no longer on the "critical path" by mid 1943. This, in turn, stems from the fact that prior to capturing the East Indies, Japan had enough oil reserves for two years of defensive warfare, but only one year of offensive warfare.

The East Indies were critical in late 1941-early 1942, because Japan was planning a major offensive that would take them across Southeast Asia and the South Pacific. Hence the pressing need for East Indies oil.

By 1943, Japan was on the defensive. Say the Allied plan was to capture the East Indies (not a sure thing in mid-1943), and that the campaign would take six months, to the end of the year. This would require not only the American forces of General MacArhur and Admiral Nimitz, but also e.g. the Eighth Army of Britain's Montgomery, and a large part of the British fleet. From the defensive standpoint, Japan could hold out at least two years to late 1945, the timetable for the later-planned Operation Olympic.

Thus, the capture of the East Indies would not have shortened the war against Japan. It might have worked against another enemy that would have surrendered after losing their oil. But not one whose generals planned to defend the homeland using "sharpened bamboo sticks."

In the actual timeline, MacArthur made it to the Philippines and Nimitz across the Pacific islands (two fine "consolation" prizes). A huge advantage of this plan was that it spared British land and naval forces that were instead sent to Italy. In the fall of 1943, it might have been a choice between knocking out Japan and knocking out Italy, except that Italy was far more "knockable." This also had the advantage of preserving the Europe first" strategy. (According to Samuel Eliot Morrison, only America's Admiral King and Britain's Lord Mountbatten, among the top Allied military, favored the East Indies strategy.

I am not sure if this is the answer you are looking for or not. The question "East Indies oil" was arguably where the Pacific War started. But that's not where it ended. Why was that?" makes it sound like you are asking why by '45 was Japan still not investing resources in obtaining Dutch Indies oil, but it could also be interpreted as why did the USA and Allies not force a final battle in the southern theater as opposed to concentrating all its forces against Japan proper.

Also, a lot of the numbers that I have seen regarding the oil used and stored are not consistent among authors which makes drawing some conclusions difficult, but by '43 Japan was able to restore Dutch Indies oil to about 75% of its former production and was obtaining approximately 50 million barrels of oil, in fact in early '43 Japan did not really have a problem with oil, in fact they may have even had a surplus.

"Within a short period, Japan was able to restore the Balikpapan oilfields with astonishing results that far exceeded their goals. Oil production in the Southern zone in 1940 was 65.1 million barrels. In 1942, the Japanese managed to restore 25.9 million barrels, and in 1943, 49.6 million barrels (75% of the 1940 level). With the East Indies oil, Japan was able to import enough oil to make up for the oil embargo in July 1941 by the Americans, British, and Dutch. There was no lack of oil, and the Japanese fleet could even refuel locally at will. They even struck a giant field in central Sumatra in the Minas structure. All these events helped make Japan feel that the oil problem, which was the driving force for its aggression, had been solved." [1]

In very late '43 and into the start of '44 they started to experience oil problems again, to alleviate this they reduced civilian oil consumption to 4% of 1940 levels and started to explore other options like increasing synthetic oil production (which never amounted to more than a couple million barrels a year) and production of pine oil, castor oil, and even sake, but this again amounted to only about 1 million barrels per year and was never a real long term viable solution as it fouled their engines up and was extremely labor intensive. They were also still able to get small amounts of oil out of Manchuria but transport and logistics of doing this were immense.

By '44 they were again in a massive oil deficit, they could only partially fill their fleet for the Battle of the Leyte gulf.

"Eventually, the urgency of fuel led the Imperial Navy to throw all of its weight into the Battle of Leyte Gulf off the Philippines in Oct 1944 as the noose was then growing pretty tight. The shortage of fuel handicapped the Japanese again and again, and the 3-day battle led to a devastating defeat for the Japanese." [1]

The loss of all this shipping tonnage greatly reduced their previous requirements for oil simply because they had less ships to put fuel in and did not need as much oil as before and the ships they did put oil in were only partially filled because they were constrained to operating much closer to the home island. So by late '44 and '45 when they were pretty much out of oil they were stuck without options. Their meager production of synthetic oils, pine oil, castor oil, etc, were enough to mount some defensive operations but it was not enough to mount another campaign down south to secure their supply lines.

I would argue that the Battle of Leyte Gulf was Japan attempting to reopen lanes to the Dutch Indies, because once the Philippines were occupied and secured traffic and communication lines to the Indies would be effectively severed. After Leyte Japan did not have the ships (merchant and military), pilots, or manpower to operate in the southern theater aside from ships that already existed there and if those ships were pulled out and sent to Japan it was a one way trip.

"' Reno I ' was based on the premise that the Philippine Archipelago, lying directly athwart the main sea routes from Japan to the sources of her vital raw materials and oil in the Netherlands Indies, Malaya, and Indo-China, was the most important strategic objective in the Southwest Pacific Area. Whoever controlled the air and naval bases in the Philippine Islands logically controlled the main artery of supply to Japan's factories. If this artery were severed, Japan's resources would soon dry up, and her ability to maintain her war potential against the advancing Allies would deteriorate to the point where her main bases would become vulnerable to capture... Strategically the Halmahera-Philippines line had been penetrated and the enemy's conquests to the south imperiled by an imminent threat of Allied envelopment. The rolling up of the remainder of this line would cut off in the Netherlands East Indies the Japanese Sixteenth and Nineteenth Armies, a force estimated at nearly 200,000 men, and would sever essential supplies of oil and other war materials from the Japanese mainland" [2]

MacArthur said in his reports on 10/25/44:

"The strategic result of capturing the Philippines will be decisive. The enemy's so-called Greater East Asia Co-Prosperity Sphere will be cut in two. His conquered empire to the south comprising the Dutch East Indies, and the British possessions of Borneo, Malaya and Burma will be severed from Japan proper. The great flow of transportation and supply upon which Japan's vital war industry depends will be cut as will the counter supply of his forces to the south. A half million men will be cut off without hope of support and with ultimate destruction at the leisure of the Allies a certainty. In broad strategical conception the defensive line of the Japanese which extends along the coast of Asia from the Japan Islands through Formosa, the Philippines, the East Indies, to Singapore and Burma will be pierced in the center permitting an envelopment to the south and to the north. Either flank will be vulnerable and can be rolled up at will." [3]

Japanese Admiral Toyoda, in after war interrogations, said as much when he stated:

"Should we lose in the Philippines operations, even though the fleet should be left, the shipping lane to the south would be completely cut off so that the fleet, if it should come back to Japanese waters, could not obtain its fuel supply. If it should remain in southern waters, it could not receive supplies of ammunition and arms. There would be no sense in saving the fleet at the expense of the loss of the Philippines." [4]

By this point getting oil from the Dutch Indies was a moot point, unless the ship was pulling up and filling right from the field, they could not get the oil to where it was needed due to the US submarine campaign, control of the sea lanes, and lack of transport ships, specifically oilers. Japan could simply not build merchant ships, specifically oil tankers, as quick as the Americans and Allies were sinking theirs. Also with the capture of the Philippines their sea lanes to the Indies were severely restricted, mainly being running in shallow waters along the coast of China where US submarines had more difficulties operating.

"The real problem was getting the oil to Japan. The Japanese tanker fleet was never adequate, and insufficient priority was given to building more tankers. Hence much of the production from Southeast Asia never made it to Japan. Although production in Japanese-controlled areas peaked at almost four million barrels a month in 1943, imports to Japan never exceeded about 1.4 million barrels a month. In the last two years of the war, the Japanese Fleet lost its advantage of interior lines of communication because of the necessity of basing much of the fleet near its fuel sources, at Singapore or Tawi Tawi" [5]

So to answer why the war didnt finish in the Dutch Indies is because by then it had become impossible, in '45 Japan could barely sail out of their home waters, they had ships stranded at home because they were out of oil, so it was already too late in the game. Their overall war strategy had changed from being a war of establishing an empire and securing vital resources to being a war of survival, and they adjusted their plans accordingly. Realizing this they chose to concentrate their remaining forces in the places they did and attempt to make the US pay as much as possible for any ground they took in an attempt to increase their negotiating power. A further attack down south was completely out of the question.

And as Mark Olson mentioned below, the Americans had no desire or need to force a final battle in the Dutch Indies, the Dutch and British islands were already isolated and "withering on the vine." It would be foolish to waste manpower in the south when the Allies had decided the final battle would be in the north against Japan itself.