Torture is illegal. Americans tortured. Why isn't anyone being prosecuted?

If you're following the revelations from the Senate's newly-released report on CIA torture, two questions might have crossed your mind. Will anyone be prosecuted for this? And if not, why not?
 The answer is that, no, there probably will be no major prosecutions for torture, at least not in the US. While torture is illegal under US and international law, most of the legal avenues that could be used to hold CIA and higher officials accountable have been closed off, one by one — many by the Obama administration. There are three main kinds of possible torture prosecutions: US prosecution for following the Bush administration's rules for torture, US prosecution for going beyond those rules, and foreign prosecution for torturing at all.
 Here's why none of those three is likely to result in any convictions.