Operation Causeway was the original plan for the Allied forces (by that I mean the US forces) to invade Taiwan instead of the Philippines as they island hop towards Japan.
Admiral Nimitz was a strong proponent of this plan, which involved the navy first taking control of Penghu, then have the Marines storm the beaches of Kending, Linn Pien, Tung Kang and Talinpu. The 10th Army would then take control of Kaohisng and military bases in Fengshan, which would act as their base as they slowly make the push towards Taipei.
They didn't go through with all the planning because MacArthur felt he made a promise to retake the Philippines. The documents were declassified in 2009.
In order to soften Taiwan's defending army, the US planned to drop retaliatory bio-chemical weapon (mustard gas) on Taipei. In the US' view point, the Formosans contributed greatly in the Japanese war effort. Taiwan provided food to supply the Japanese troops, provided important ports and airfields, that allowed the Japanese to strike targets all around East Asia, and volunteers filled Japanese ranks.
In fact Taipei was on the short list of US' atom bomb targets as well. Had they went ahead with Operation Causeway, and the mustard gas worked well in Taipei, they would also drop mustard gas on other major Taiwanese cities. Dropping mustard gas on Taiwan would be a practice for future operations, such as Operation Downfall and Operation Olympic, when Allied forces invades Honshu and Kyushu.
As previously mentioned, Taiwan escaped this fiery (and poisonous) fate because MacArthur insisted "I said, to the people of the Philippines whence I came, I shall return. Tonight, I repeat those words: I shall return." He felt that the US had colonized the Philippines for half a century, and its people were eager to assist their efforts. MacArthur and Nimitz fought for at least 4 months on this issue. Lieutenant general Simon Bolivar Buckner of the 10th Army supported MacArthur, and Admiral Ernest Joseph King of the navy supported Nimitz.
By Oct. 3rd of 1944, the US decided to target Luzon instead of Taiwan. In order to prevent fighters from Taiwan to hinder operations in Luzon, the US first conducted the Formosa Air Battle. US planes heavily bombed Taiwan and Okinawa, and destroyed Japan's air capability in the region.
The 10th Army eventually went on to attack Okinawa instead, where general Buckner died from a shrapnel, becoming the US' highest ranked officer to get KIAed in WW2.