To give you some data point with regard to enforcement, my SO has a Japanese passport and acquired US citizenship. No issues entering/leaving Japan after acquiring US citizenship. Japan technically allows only Japanese citizenship past age of 20.
I also have a friend that was born with both US and Japanese. Didn’t have to pick. However, US embassy indicates that even if you notify Japan that you pick Japanese and renounce other citizenships, it is in fact a very formal process (almost difficult) to lose US citizenship.
It’s quite irregular. As an American abroad, we get zero awareness or sympathy from people „back home“. They either assume it is normal among countries or that it doesn’t exist.
Taxation treaties only go so far. Declaration overhead, schedule category, and tests make the process hellish. And I haven’t even begun to mention the FBAR and other disclosures. It hurts everyday Americans abroad and barely differentiates against real versus nominal income and cost level.
Yes, even if you are entirely overseas. The US and Eritrea are the only countries in the world that tax all world-wide income. As long as you are a US citizen, you can live anywhere else in the world for however long you want, and you are still taxed in the US and have to file and pay taxes every April.
Not only that, you are required to disclose all of your foreign bank accounts and their balances on a special "FBAR" form to the FinCEN (separate agency from the IRS). Penalties for any "violations" start at $10,000 each (for so-called "non-willful" violations such as using the wrong currency conversion rate or a different bank branch address) and go up to 25% account value for willful violations (everything is conveniently considered "willful" whenever possible to FinCEN in absence of fact, even the aforementioned common mistakes). Many expats get screwed because they simply didn't know about this esoteric form. Hiring a professional to properly prepare your FBAR with their signature on it is necessary as a form of "insurance" (you are welcome to roll the dice and self-prepare), and of course, it is also an expensive proposition.
There is also no clear guidance for what the disclosure requirements are for foreign cryptocurrency exchange accounts or wallets. Anyone living inside or outside the US with a Binance account could get slammed at anytime in the future for not submitting an FBAR.
If before leaving the US, you lived in California, you also have to pay California state taxes on all world-wide income every year, since you are still considered a California "resident."
There is a foreign earned income exemption up to around 100k. This applies to ordinary income only, not capital gains (e.g. cryptocurrency), and you have to meet rather-stringent requirements to qualify. Also, the exempted income still pushes up your capital gains bracket to the highest applicable rate. This exemption does not apply to your California taxes, only your federal taxes.
You almost never hear about how disfavorable the US tax laws are for expats because there are only 8 million of us.
Foreign banks are required to report information about accounts owned by US citizens to the FinCEN (this is called FACTA). You may remember being asked if you are a US citizen when you opened your foreign account -- this is why. If the banks don't provide this information to the US government, then they are heavily sanctioned. Many criticize FACTA for its world policing, and past attempts at repealing it, spearheaded by none other than Rand Paul, have been struck down by the courts.
If something doesn't match up between what the banks reported about you and what you reported, then this triggers an audit. Then, the IRS will request documentation, and if you do not provide it or if it does not match, then you are criminally charged with tax evasion, facing heavy fines and jail time.
This whole sour experience of being an expat and getting screwed in multiple ways has literally turned me into a tax reform lobbyist. It's quite a different experience than importing your W-2 into Turbo Tax in 30 minutes and getting a refund every year like 99% of Americans.
Yes. Even if you've been gone 10 years and never visit and never plan on returning. Some folks find they technically need to pay tax because their parents are American - even though they've never stepped foot on American soil.
There are exceptions for local taxes, but you still have to file tax returns.
I also have a friend that was born with both US and Japanese. Didn’t have to pick. However, US embassy indicates that even if you notify Japan that you pick Japanese and renounce other citizenships, it is in fact a very formal process (almost difficult) to lose US citizenship.