This is the official USG policy on dual nationality.
Berg argues that by the Hague convention the US, as a party, has agreed to respect other countries laws regarding citizenship. In particular, if another country prohibits dual citizenship and you become a citizen thereof, then you necessarily become a non-citizen of the US.
This just simply isn't so. We have millions of Americans who hold dual nationality in this country, including holding passports from those countries, e.g., Israel, Ireland, etc.
"A U.S. citizen may acquire foreign citizenship by marriage, or a person naturalized as a U.S. citizen may not lose the citizenship of the country of birth.U.S. law does not mention dual nationality or require a person to choose one citizenship or another. Also, a person who is automatically granted another citizenship does not risk losing U.S. citizenship. In order to lose U.S. citizenship, the law requires that the person must apply for the foreign citizenship voluntarily, by free choice, and with the intention to give up U.S. citizenship."
Policy is not law and Berg believes the Hague Convention requires the US to respect the law of Indonesia in preventing dual nationality. I guess Berg’s contention here would be that so long as it doesn’t contradict the Constitution, treaties are law, even if they don’t appear in the statute books.
I don’t know whether dual nationality is prohibited by Ireland or Israel, so it may not apply there. I’m virtually certain Israel doesn’t prohibit dual nationality, since a significant fraction of the population openly votes in US elections. It could also be that Berg is right, but nobody previously felt like pressing the issue.
I don’t believe Berg is right, but the law is sufficiently complex and I am sufficiently ignorant that I admit that he might be. Frankly, I think Berg is throwing everything up against the wall that he can imagine to see if anything will stick.