Is it possible for an Australian to become an American historian?
yes of course. And you don’t have to live in the US in order to do it. But if you want to I would recommend getting an undergrad degree at an Australian university and then go to graduate school in the US. The reason being that an university education is very expensive in the US and tuition fees can run to $20 000 – $50 000 a year. If you are really good as an undergrad you might get a scholarship as a graduate student or your school may give you a job teaching, so being a graduate student can be more affordable.