I read this excellent post by /u/thefourthmaninaboat regarding America's "Rainbow" war plans during the 1920s, and specifically War Plan Red. War Plan Orange, a strategic plan for war with Japan, was also mentioned, however, and as I'm not particularly savvy about American history it's the first I'd heard of it. Almost two decades passed between the early work on these strategic plans and the eventual declaration of war between Japan and America, and during that time undoubtedly a great deal of new military hardware and tactical innovations became available to the USN which would quite probably have made War Plan Orange outdated on more than just its assumptions regarding Japanese strength and territorial breadth. Nevertheless, were there any holdovers from WPO, strategic or otherwise, which were retained in part or whole during the war in the Pacific?
Plan ORANGE, in the end was incredibly predictive in both large scale form, and specific details in how the US waged war on Japan. The form the plan took by its incorporation into the larger RAINBOW schemes had adapted just about all the technological and tactical changes through the late 30's, and in some cases was the instigator of them.
Now there is some 'chicken or the egg' to be fair, in that the men who were charged with directing the war effort were long steeped in the plan and its evolution. Men like Admirals King, Nimitz, Kelly Turner, and Spruance had all spent years first as students, and in many cases as instructors. ORANGE was in essence less a plan then a paradigm and group of collective judgements of American Naval leadership for fighting their most anticipated potential foe.
So lets sketch how it stood int he late 30's. The plan had 3 phases.
Defend the Pacific midpoint/IDL, or if needs be Hawaiian frontier. Spend 6-12 months if needed in getting the fleet on a war footing and bring more forces online. Also defend the sea lanes to positions in Samoa/Fiji, and the ANZACs if allied and in the war. Harass the enemy as possible, namely in the Mandate islands using mixed forces of light units of carriers and cruisers, backed up by the battleships should enemy heavy forces appear. Despite the hopes of the Army, and some aggressive planners the Philippines here were essentially assumed to be a lost cause.
Was the push across the central Pacific. Starting in the Mandates, possibly the Marianas or Palau, Guam if it had fallen, and eventually establish a large forward base in the Southern Philippines. Ideally the chain of seized islands would allow for the isolation of any enemy islands to the rear and serve as ports for forces to protect supply lines from raiders. Also at some point during this phase hopefully a sufficiently important target was hit that the IJN was forced to sortie for a fleet battle to damage them, and put them further on the back foot.
Then was the most nebulous. With the Philippines at least US dominated if not retaken outright then the fight would move to Japanese homewaters. A fleet action would be fought if needed and the Japanese govt battered into surrender, mostly by blockade, most versions did not contemplate an invasion of the Home Islands.
The similarities of the plan to the US Central Pacific strategy should be clear. Certain facets like turning Guam into an America Gibraltar, rushing forces to the Philippines to open the war, or sending off carriers to aid potential British Allies were all dropped for one reason as another. While planning groups absolutely understood the need to have a huge fleet train of support vessels and were in the right mindset to assemble that from rough drafts when war came. And cross service the USMC was deeply immersed in the interwar in studying the challenges of amphibious assault and the doctrine they developed informed every assault on a beach made by American forces during the war. The need for specialized amphibious vehicles, proper study of landing spots, properly coordinated gunfire support, and a readied support element to reinforce the beach were all aspects developed here.
On some of the major specifics many of the major islands had been examined as options years before. While Truk was understood as great, Ulithi and Eniwetok were also looked at as alternatives for bases in Phase 1 that became major fleet anchorages. And with US naval dominance ahead of schedule by reaching the Philippines the planned landing spot on Mindanao was moved North to Leyte and paired with a recaptured Guam to be the major logistics bases to support Phase 3, joined by Okinawa later.
Air power was absolutely incorporated into the plan first as a scouting tool in its early days. But the ability of a carrier to swing a battle or land based air to attack invasion fleets was not doubted at all even by the most old school admiral. And many of those who had spent the most time shaping ORANGE were naval aviation supporters. And the procurement strategy of the USN in the late 30's early 40's prewar reflected this incorporation. Other branches like submarines also developed to meet the needs of a war fought between islands huge distances across the Pacific.
War Plan Orange by Edward S Miller is absolutely a fantastic source on the evolution of the scheme and how its particulars or spirit was applied.