Meir Dagan has been described as "hard-charging" and "stops at nothing." For more than eight years, Dagan made full use of those qualities as chief of Mossad, Israel's intelligence agency, where he focused on keeping Iran from developing a nuclear weapon. When that job ended, Dagan did something unheard of for an ex-Mossad chief: he spoke out publicly, voicing opposition to Israel launching preemptive airstrikes against Iran's nuclear facilities anytime soon. Dagan believes the Iranian regime is a rational one and even its president, Mahmoud Ahmadinejad - who has called for Israel to be annihilated - acts in a somewhat rational way when it comes to Iran's nuclear ambitions. Lesley Stahl reports.

The following script is from "The Spymaster Speaks" which aired on March 11, 2012. Lesley Stahl is the correspondent. Shachar Bar-On, producer.

When President Obama met with Israeli Prime Minister Benjamin Netanyahu this past week, the subject was how, when and if to attack Iran's nuclear facilities, Netanyahu saying Israel can't afford to wait much longer; Mr. Obama arguing there's still time to let sanctions and diplomacy do the job. And he said some top intelligence officials in Israel side with him.

Actually, you'll hear from one of them tonight: Meir Dagan, former chief of the Mossad, Israel's equivalent of the CIA. It's unheard of for someone who held such a high-classified position to speak out publicly, but he told us he felt compelled to talk, because he is so opposed to a preemptive Israeli strike against Iran anytime soon.

Dagan headed the Mossad for nearly a decade until last year. His primary, if not his only mission was to prevent Iran from developing a nuclear bomb. And he says there is time to wait, perhaps as long as three years.

Lesley Stahl: You have said publicly that bombing Iran now is the stupidest idea you've ever heard. That's a direct quote.

Dagan: An attack on Iran before you are exploring all other approaches is not the right way how to do it.