Even if the Iranian leadership were not always rationally interested in self-preservation, Israeli missile defense, second-strike capability, and a careful understanding of Tehran's priorities could potentially keep a nuclear Iran at bay.
Whatever their current disagreements on the reasonableness of further sanctions, Washington and Jerusalem have now both indicated that they may be willing to use military options against Iran. After all, sanctions have apparently not convinced Tehran to abandon their nuclear efforts, and rejecting the military option altogether could leave Israel vulnerable. At the same time, what would have to be a stunningly complex and operationally unprecedented preemption, and at this conspicuously late date, could easily result in catastrophic failure.
Assuming that nothing else works, then foregoing a defensive first-strike would mean that Israel would have to live with the protracted uncertainty of a potentially nuclear Iran. Unless Iran's leadership remains consistently rational, thus valuing its own national survival above all else, then Israeli nuclear deterrence would be only more or less effective. With this likely in mind, Israel has been expanding and upgrading its network of active defenses.
An improved Israeli interceptor called "Block 4" contains new software designed to intercept Iran's Shahab and Sajil missiles. Israel is also correctly concerned about Iran's newest version of the Conqueror rocket. The centerpiece of Israel's active defense plan for Iran remains the Arrow anti-ballistic missile program, which was recently tested successfully. Iron Dome, a complementary system, is intended primarily for intercepting shorter-range rocket attacks. Still under development is David's Sling, which would be intended for use against medium-range rockets and cruise missiles. Still, no system of ballistic missile defense could ever be so reliable as to preclude a corollary strategy of deterrence, especially if the incoming warheads were biological or nuclear.
Read more at The Atlantic.