The problem is that by 1918 the US already had everything in the Pacific Rim that it wanted. Open Door in China, Philippine Islands, Guam, Hawaiian Islands, Samoa and a scattering of other islands.
There was the real fear of war with Japan in the 20s and 30s, but I don't believe a first strike was ever contemplated. About the only thing I can think of is if the Japanese ruthlessly murder a large number of Americans and Europeans, perhaps missionaries in China. And then the US gains word, through broken Japanese codes, of an impending attack. This could be enough for the US to strike first to end the threat.
Benjamin