I’m aware of War Plan Orange and the post-WWI planning, but wasn’t Japan a nominal member of the Allies during the first war, before they brought in the bugfuck crazy Black Dragon regime? Doesn’t mean there weren’t rumblings among US war brass that indicated, “hey, we might have to go to war with these guys if they pose a threat in the Pacific, and they’re pretty hardcore - did you see what they did to Russia?” but a lot of that had to kind of ebb and flow before the interwar period and Japan deciding to take matters into their own hands and strike first.
I don't have my sources on me right now, but I'd just figure I'd list it for what I remember.
While Japan was much a part of the Entente, it didn't actually contribute much to the warfare (no soldiers on the Western Front for sure), and it was of the opinion of the brass that the only nation that could threaten the US majorly was a nval opponent. Chiefly Britain, although that did warm as years go by. Germany is an understated one, given their desires for parity with the British. But the Japanese, with their obvious dissatisfaction with the outcome of the Russo-Japanese War and how they laid the blame on the American's feet, were quite aggressive and were quickly expanding their own Navy to establish dominance in the region. Add to that continued Japanese interest in dominating China, and they were, very early on, becoming interested in consolidating their position in Korea and projecting power into Manchuria and other neighboring regions.
Japan was the rising power in the East, and the only Great Power that actually was located on the Pacific (the other being the US) as the British Empire's most important priority was Europe.
It's why the Anglo-Japanese Alliance's end was one of the conditions of the Washington Naval Treaty, as American Naval planning treated them as one and the same, with Japanese fleets being essentially British auxiliaries, hence military construction would be necessary to combat both Japan and Britain at once.
So, while the US weren't as directly worried about a Japanese threat to the US possessions, it was always something they prepared for. War Plan Red-Orange is the child of war plans and contingencies that had been ongoing for decades by that point, merely codified.
I'm probably off on some exact points, but that's the general understanding. The US-Japanese antagonism didn't erupt over a decade or two. It's a process that, I feel, is the result of two younger powers reaching out, with their regions of interest overlapping and clashing, which slowly grew worse and worse over time.