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I think it was always obvious that at some point they would become a super power, they are just that big. In Hitlers second book he mentions it a lot. It's part of the reason he thinks Germany has to expand, in order to compete with the US which has a larger size and population. It's also why he talks about allying with Britain and also why some say he didn't destroy their army at Dunkirk. In his view were Britain to fall the US, USSR or Japan would fill their role.
But maybe people did not think it would happen so soon.