Though let's be real, having the wealthiest and most powerful nation on the planet as your Ally validates the reason and purpose of defensive pact.

Figure that Japan would be a member by default alongside South Korea and depending on the extent of area OZ and NZ. Past that a slightly harder, but likely a reasonably "easy sell" would be to the Philippines...

That would make a hell of an Asia pacific block of economic and military might.

Vietnam, Cambodia, Laos... i'm sure would love to get assurances against Chinese aggression over all even though due to historic reasons they might be against being that chummy with the US. However, there is general dislike of one, and ones recognized need for protection against another. Which being said how good of a deal can one make such an arrangement to be to get such states to join in?

Tons of other nations though whose positions i have little to no idea of though.