You can teach in the United States, you only need the proper documents as you do in any country....

And the USA doesn't require any special Cambridge certificate.....

and no Harvard ELTA certification needed.

Equal opportunity

You need:

1. A work visa
or

2. A green card

or

3. US citizenship

you can apply for a tax payers id (SS number) without a green card or citizenship.

So yes, you can come and teach in the USA....