You can't train overseas and "just" get a license in the USA. Even if you've been practicing as a dentist in another country, to practice in the USA one must attend dental school in the US and pass the board licensing exams. It may be that some states allow taking only 2 years of dental school instead of the normal 4 if one has been a dentist elsewhere.

I've found that many people are surprised when they discover that dentists in the USA are medical doctors and have a similar education.