In my view, sending a letter in the course of the nuclear negotiations is not a big deal. If one believes that negotiations (including sanctions) are the way to resolve the nuclear standoff, then this letter is par for the course. But if military attack is the way to go, then sending a letter is a sign of weakness

Furthermore, this letter was sent a few weeks ago when ISIS problem first surfaced and Zarif wanted to tie the Regime’s cooperation in fighting ISIS with the nuclear negotiations. Obama rejected the linking of the 2 issues then and this letter could be the official US response to Khamenei’s offer of linkage

At the end of the day, what really matters is the content of the final offer to the Regime and if an agreement can be reached

Never negotiate with a filthy rat like Khamenei. The only thing he needs is a bomb in his throat.