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How can we reach a situation in which in addition to the US, other nation(s) are operating carriers similar in capability and size to at least the Forrestals?

I think here that the most likely canditates are the UK and Japan. With Japan, the scenario would have to avoid World War II or any other conflict that could lead to the destruction of the Japanese Empire. With Japan wanting to keep their carrier fleet strong in the face of the US, I could see them eventually advancing to building supercarriers, although definetly not as many as the US.

The UK could probably have the capability to build them if they decided to put an effort towards it, but it's harder to get a reason for the UK to build supercarriers with the US being a major ally. Possibly the Suez Crisis being resolved favorably towards the UK leading to them to maintaining a larger world presence.
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