From Time, Obama: Prevention, Not Containment (March 5, 2012)

In photo-op before closed-door meeting Monday morning, Obama tells reporters his commitment to Israel's security is "rock solid," says the U.S. will focus on stopping Iran from obtaining a nuclear weapon.

POTUS: "My policy here is not going to be one of containment. My policy is going to be of prevention."
I'm convinced the shift from "containment" to "prevention" was well thought out; and that the President is well aware of this spectrum:

Containment > Responsive War > Preemptive War > Preventive War

and what "Preventive War" means:

A preventive war or preventative war is a war initiated to prevent another party from attacking, when an attack by that party is not imminent or known to be planned. Preventive war aims to forestall a shift in the balance of power by strategically attacking before the balance of power has a chance to shift in the direction of the adversary. Preventive war is distinct from preemptive war, which is first strike when an attack is imminent.
Someone else can suggest the various options for waging "Preventive War". I'm still in "Containment > Responsive War" mode; and intend to remain there. And, no, a back and forth between Israel and Iran would not justify an armed response by the US against Iran. But, I don't write the US playbook.

Regards

Mike