That is an excellent question, one many keep asking, considering we have Iran's despotic Islamic régime surrounded from Afghanistan, Iraq, along with our Navy in the Persian Gulf. The only border left to Rat Boy and the Mullahmania is the Caspian Sea, and further to the north, Putin, feeding the Iranian dictatorship all the nuclear fuel they require, Russian anti-aircraft installations protecting their nuclear weapons plants, and Russian nuclear technicians making the unthinkable a dangerous reality.
Now Israel is stuck, alone, with a very difficult task, again (the first time when they took out Saddam's French built nuke weapons plant), since what lurks in the White House supports the enemy in Tehran.