The letter drafted by the freshman Senator Tom Cotton, and signed by 46 of his Republican colleagues, warning Iranian Supreme Leader Ayatollah Ali Khamenei that any deal he makes with U.S. Non-Supreme Leader Barack Obama might soon be undone by right-minded American legislators, is problematic on a number of fronts. But let me address just a couple, in the interest of time (I'm off to Germany to give a couple of talks on—of all things— the manner in which American foreign policy is made), and because others (including Dan Drezner) have addressed some of the broader issues already.
Here are two things I think are true about the current Iran nuclear negotiations:
1) These talks give the West the best, and least bloody, opportunity possible to keep the Iranian regime far from the nuclear threshold;
2) There is still a decent chance that Ayatollah Khamenei will ultimately balk at the conditions set by the U.S. and its allies, and wind up subverting the talks. Remember, as weak as this still-theoretical deal seems to some people (I'm in the camp of the moderately worried on this front), the conditions laid down by the West may be seen by the viciously anti-American ayatollah as too humiliating to accept. For instance, the deal might break apart over the issue of sanctions relief. The Iranians want to see most sanctions removed immediately upon completion of the deal; the West, of course, wants to phase out sanctions gradually, in order to ensure Iranian compliance with the terms of the deal.