President Obamamust home in on the foreign policy challenge that poses the most imminent threat to world security: Iran. The theocratic regime in Tehran is undeniably closer to obtaining a nuclear bomb. With that weapon in hand, the regime will have the ability to deliver a devastating blow to the United States or any one of its allies. With a "nuclear umbrella," Iran could also continue to terrorize the world with impunity, thanks to a state-run terror apparatus known as the Islamic Revolutionary Guard Corps, as well as proxies like Hezbollah in Lebanon, Palestinian Islamic Jihad in the Gaza Strip, and still others elsewhere. The president must push forward with punishing sanctions, and credibly threaten the use of force. Should Iran choose not to step back from the nuclear brink, the president may also need a feasible plan for military intervention. If Iran builds a nuclear weapon, it will almost certainly happen on his watch.