From wikipedia
The origins of the battle lie in the complex politics of the Pacific theater, in which operational control was divided between the South West Pacific Area (command) of General Douglas MacArthur and the Pacific Ocean Areas (command) led by Admiral Chester Nimitz. The potential for interservice rivalry between the Army and Navy created by this partition of responsibility was exacerbated by similar divisions within the Joint Chiefs of Staff (JCS) in Washington. By September 1944 the two servicea could come to an agreement about the main direction of advance towards the Japanese home islands in the coming year. The Army was pressing for the chief effort to be an invasion of Formosa (Taiwan), in which MacArthur would be in overall command and in which it would predominate. The Navy however preferred the idea of an operation against Okinawa, which would be a mainly seaborne effort. Seeking to gain leverage and so break the impasse, on September 29 Nimitz suggested to Admiral Ernest King that as a preliminary to the Okinawa offensive the island of Iwo Jima could be taken.[2] The tiny island lacked harbors and so was of no direct interest to the Navy, but for some time General Henry Harley Arnold of the US Army Air Force had been lobbying to take Iwo Jima. He argued that an airbase there would provide useful fighter escort cover for the B-29 Superfortresses of his XX Bomber Command, then beginning its strategic bombing campaign against the Japanese home islands (the later role of Iwo Jima as a refueling station for B-29s played no part in the original decision-making process). Arnold's support in the JCS enabled the Navy to get Okinawa rather than Formosa approved as the main target on October 2.[3] At this time the Iwo Jima invasion was expected to be a brief prologue to the main campaign, with relatively light casualties; King assumed that Nimitz would be able to reuse three of the Marine Corps divisions assigned to Iwo Jima for the attack on Okinawa, which was originally scheduled to take place just forty days later.
So, could there have been an amphib invasion of Formosa under Mac's direction instead of against Iwo in Jan 1945 ? What would've been the consequences of such a move on the overall scheme of things in the Pacific ?