Let's keep in mind that it's traditionally been the Japanese government unwilling to pay for a large military no matter how much the US government has pushed them. The "no rearming" bit didn't last for very long.
Hmm….
Ok. POD. IOTL the US abandoned Japan halfway through the largest cultural re-engineering project ever attempted. They hared off, to go fight Communism, and left the thing half done, unfinished, deeply weird.
In the alt-timeline the USA stays, but with a slightly different purpose. Instead of leaving to go fight communism they (and, presumably, also with Germany) take a different tack: they stay, in order to create proxies to fight communism for/with them.
Perhaps this is because of a more pro-active China position, thus requiring a stable contributing Japan earlier. With a broader spectrum of American advisors the KMT manages the economy somewhat better, and demobilizes somewhat slower.
This weakens the Communists, in turn making the staying involved position more attractive for America (i.e. weaker ChiComs means the US sees it as winnable, instead of hopeless). However the USA is busy demobilizing themselves, and so they begin to shift position regarding Japan. Meanwhile the USSR is busy taking over their Chinese commie comrades as much as possible as their weaker position means they are far more dependent than OTL.
A little while later the Japanese are involved in naval and air operations against the ChiComs, the US remains deeply embedded continuing their cultural re-engineering, and so forth.
Taiwan flares up as the Nationalists are weakening and the Japanese subtly demand it back. They point out that the US can't literally make them fight against the ChiComs, and perhaps the domestic pacifist forces are gaining in power.
The US is just realizing how messy this whole thing is, but they're too deeply involved to pull out now. So the ROC gets a big new infusion of cash and guns, and the Japanese take over Taiwan again.
However the US does want to formalize this, so they come up with the Asian Pacific Treaty Organization (APTO). Incorporating Australia, New Zealand, the Philippines, ROC, Korea, and Japan into basically the anti-communist alliance.
In return for yet more American cash and guns the whole thing comes together, not least because the Nationalists keep losing (ITTL they kept the economy together, but they're still pretty incompetent) and the USSR and their presumed ChiCom proxies are becoming more and more scary.
This goes on for a few years, until it settles down (either the Nationalists or ChiComs win, or they de facto divide up the country to whatever extent seems reasonable).
So. Japan is rearmed, actively fighting communists. In return, the US continues to screw with Japan and god knows what comes out the other side (half-finished, they produced a pretty darn weird Japan IOTL). There's a NATO analogue Asian alliance. In return the ChiComs are pretty dependent on the USSR and could come with an Asian Warsaw Pact if they wind up with a North Korea, or if India swings commie later on.
Eh. Probably not that plausible, but it was fun

.