If you are a US citizen or green card holder, you file a US tax return and pay US tax on all your world-wide income, no matter where the income is from or where you are living.
If you surrender your citizenship, or surrender your green card, you are a non-resident alien. (This also applies if you never were a green card holder or citizen, of course). As a non-resident alien, you file a special US tax return (1040-NR) that only pays US tax on "US-sourced income". Pensions, including 401k plans, are taxed based on where you were living when you did the work. That means that your 401k withdrawals will be taxed by the US no matter where you live in the world and no matter where you are a citizen. The withdrawal may also be taxed by your home country, but that would depend on their rules, and whether or not they give you a break for being taxed twice is up to them. If the US has a tax treaty with the other country, that might change how your 401k is taxed, you would have to let us know the other country and someone could try and look it up.
As a non-US citizen, the 401k trustee is required to keep mandatory withholding of 30%. When you file your non-resident tax return, if the withholding is more than the tax you owe, you get the difference back as a refund.
