Given that Iran already has the bomb, an Iranian ship or submarine, or an Iranian-financed ship from some third country, if it gets within 1,000 miles of the east coast of America, can fire two KH-55 nuclear tipped cruise missiles programmed to fly beneath our radar horizon. The missiles can reach New York City and Washington D.C. in less than two hours. Struck by the equivalent of 12 Hiroshima bombs, each city and its suburbs will be leveled.
To the best of my knowledge, we have no defense against such an attack. Our Patriot anti-missile system is designed to shoot down high-flying ballistic missiles, not low-flying cruise missiles. During the 2003 invasion of Iraq, the Patriot system was defeated by a relatively primitive CSS-C-3 Silkworm cruise missile.
If the attackers scuttle their ship in deep water and commit suicide, we will not know for sure who hit us. (Such martyrdom is an ingrained feature of Iranian Shiite culture.) If we don't know who hit us, we may fail to counterattack, or worse, attack the wrong country. With all or most of our corporate, government, and military leaders dead, our ability to respond quickly will be compromised. With our two most vital hubs gone, we could begin to starve in weeks.
I see no reason why this attack could not take place today.