This is from the CDC (Center for Disease Control)'s website:

Domestic dogs must be free of evidence of diseases communicable to humans when examined at the port of entry. If the animal is
not in apparent good health, further examination by a licensed veterinarian may be required at the owner's expense.

Dogs must be vaccinated against rabies at least 30 days before entering the United States. This requirement does not apply,
however, to puppies less than three months of age or to dogs originating or located for at least six months in areas designated by
the U.S. Public Health Service as being rabies-free.

The following procedures pertain to dogs arriving from areas that are not free of rabies:

A valid rabies vaccination certificate should accompany the animal. This certificate should be in English or be accompanied
by a translation. It should identify the animal, the dates of vaccination and expiration, and be signed by a licensed
veterinarian. If no expiration date is specified, the certificate is acceptable if the date of vaccination is no more than 12
months before the date of arrival.

If a vaccination has not been performed, or if the certificate is not valid, the animal may be admitted if it is confined
immediately upon arrival at a place of the owner's choosing. The dog must be vaccinated within four days after arrival at
the final destination, but no more than 10 days after arrival at the port of entry. The animal must remain in confinement for
at least 30 days after being vaccinated.

If the vaccination was performed less than 30 days before arrival, the animal may be admitted but must be confined at a
place of the owner's choosing until at least 30 days have passed since the vaccination.

Young puppies must be confined at a place of the owner's choosing until they are three months old, then they must be
vaccinated. They must remain in confinement for 30 days.