At one point in his op-ed, Posen writes:

Some worry that Iran would be unconvinced by an American deterrent, choosing instead to gamble that the United States would not make good on its commitment to weak Middle Eastern states—but the consequences of losing a gamble against a vastly superior nuclear power like the United States are grave, and they do not require much imagination to grasp.

The Iranian mullahs may not be suicidal lunatics, but is Posen meaning to argue that they're incapable even of gross miscalculation? Saddam Hussein gambled twice that American presidents—both named Bush—wouldn't fight his army. Osama Bin Laden gambled that the second Bush wouldn't respond to the attack on the World Trade Center. Nikita Khrushchev gambled that John F. Kennedy wouldn't resist Soviet missile bases in Cuba. The Japanese gambled that Franklin D. Roosevelt wouldn't strike back after Pearl Harbor.

Especially if the United States pulled out of Iraq, as Posen advises, would it really be so outrageous for Iranians to gamble that America might not "make good" on some commitment, especially if their gambling aggression were limited and piecemeal? (I am not saying this is an argument against withdrawing from Iraq; I agree with Posen's position on that score.)