Just How Credible Is Israel?

"You! Yes, YOU behind the
bikesheds! Stand still, laddy!"

As Iran makes more rabid ruckus about its nuclear aspirations (er...
for peaceful purposes!), there's also growing talk about a pre-emptive Israeli strike.
An air strike against Iran's nuclear capabilities would not be unique: Israel did it before (against Iraq in 1981 and Syria in 2007), and could do it again. In fact, it's been talking this way about Iran for a number of years now...but soon it may back itself into a corner with only one way out: it would destroy its credibility if it was "the country that cried wolf".
So, is there time left for non-military options? Iran's already been slammed with four UN Security Council sanctions, and the European Union hit it with an oil embargo and an assets freeze. Hard to tell if these are working, judging by Iran's threat to starve Europe of ALL oil.
But as we know, there're many ways to skin a cat. In 2011: the Stuxnet cyber-attack (believed to have been developed by the US) that severely damaged Iran's vital nuclear centrifuges. Several Iranian nuclear scientists have mysteriously died (no, let's be honest - been murdered) over the last year. Then tit-for-tat last October, a foiled Iranian plot to assassinate the Saudi ambassador in USA. Last week: failed attacks on Israeli diplomats in Georgia, India and Thailand, and this week another busted plot against Israeli officials.
Critics of an Israeli strike say a military operation would destabilise the Middle East, but how more unstable would it be if Iran got a nuclear bomb? And while Iran's nuke programme couldn't be eliminated in a single strike, it would deliver a powerful message of what more might be in store.
Of course, Iran would strike back. Israeli officials have calculated possible scenarios and decided a certain level of collateral damage would be acceptable: "Is 40 missiles on Tel Aviv nice? No, but it’s better than a nuclear Iran."
Recently the Pentagon conceded it did not have "perfect visibility" regarding Israel’s military capacities...in other words, it didn't know quite what Israel could do. And last month analysts said Israel did not have the ability to splat Iran's nuclear plants...
Well, if that's really so, then why is everyone getting so worried?