How do US citizens get credit for PIT paid in Thailand

[JohnnyBD]

How would a US citizen who paid income taxes in the US, get a tax credit from the IRS if they had to pay taxes in Thailand on the same income? For example, a US citizen (Thai tax resident) remits 2MM THB to Thailand (all assessable monies), and pays income taxes in Thailand on that 2MM THB. I can't see the IRS just giving a dollar-for-dollar tax credit for taxes paid in Thailand without knowing the make-up of those monies, whether they were previously taxed in the US, or not taxed in the US, and from what tax year those remitted monies were derived. For example, if they were non-taxed monies from a Roth IRA, why would the IRS give a tax credit if those monies were not taxed in the US. I would think the IRS would want to know the make-up of the remitted monies and in what year they were derived. Sounds pretty complicated to me.