Since its reincarnation in 1948, Israel has been surrounded by hostile neighbors. Considering it was carved out of what was then predominantly Palestinian Arab, Muslim territory and displacing its people, that hostility is understandable.
It was not reborn without strife and resistance and wars with their immediate neighbors, Egypt, Lebanon, Syria and Jordan. One could say it was reborn in a state of siege.
However one war was transformational, the successful 1967 war primarily against Egypt, Syria and Jordan whereby Israel annexed the Sinai from Egypt, The Golan Heights from Syria and came to occupy the West Bank and Gaza and establishing Jerusalem as its capitol (that formerly had been under Jordanian jurisdiction). Overnight Israel became an occupying power primarily asserting its power in the West Bank and Gaza, by Jews (mostly Orthodox) expropriating Palestinian lands and establishing Jewish settlements while protected by the Israeli Defense Force (IDF). These forcibly taken settlements remain at the heart of the ongoing Palestinian struggle and resistance to the Israeli occupation.
Since the 67' war Israel has become, (primarily with the financial and military backing of the U.S.) the preeminent power in the region, has the strongest military and has nuclear weapons.
It is unchallenged, yet its recent actions (the military assault on Gaza and its blockade, the flotilla incident in international waters) reveal a siege mentality that Israel has never cast aside (from the time it was real and legitimate in the early days of its existence) despite its military superiority and firepower to successfully defend itself against all challenges.
And feeding into Israel's current siege mentality most prominently is Iran with its bombastic President Ahmadinejad denying the Holocaust and wants Israel wiped off the map. That Iran has no capability to achieve this end (now or in the future) and is just the ranting of a man not having the decision making authority in Iran (which belongs to ayatollah Khomeini) seems to have no bearing. Just the idea of Iran attaining a nuclear capability has the State of Israel in high alert. It is this state of readiness, its sense of imminent demise at the hands of Iran that has the country and its leadership contemplating and planning for a pre-emptive aerial attack on Iran's nuclear facilities even though Iran has no nuclear bomb making capability at this time.
At present Israel waits to see the results from the latest round of sanctions imposed on Iran by the U.S. and the West to see if that will make them cede their nuclear ambitions. The view here is Israel puts no trust in sanctions having any effect on Iran. The worry then is Israel, in their siege mentality, will act unilaterally, bomb the Iranian facilities without warning (or the approval by the U.S.) with no interest on the disastrous consequences its attack on Iran would likely bring about.
In all likelihood Israel's attack on Iran would be catastrophic i.e. the Strait of Hormuz would likely be blocked by Iran in retaliation sending oil prices skyrocketing. The U.S. forces in Iraq would come under attack by Iraq's Shiite militias aligned with Iran, further disrupting any chance of political reconciliation in Iraq. Israel would come under attack by Hezbollah rockets into Israel proper. Iran would send conventional missile warheads into Israel. Terrorism and suicide bombings would undoubtedly occur worldwide against American outposts, American's in general and Jews everywhere (as nobody would believe Israel would have acted without U.S. prior agreement). A world wide economic calamity would likely ensue springing from the disruption in oil supplies. A Palestinian uprising in the West Bank would likely erupt.
Though Iran's present nuclear facilities would be damaged it would most probably work at full speed to not only repair the damages but move expediently to acquire a nuclear weapons capability as a defensive measure.
The Israeli assault will also bring unexpected consequences that could not be foreseen before an attack.
To this observer, there is only one person capable of averting this impending disaster from happening, Barack Obama. Only he can tell the Israeli leadership that attacking Iran is completely unacceptable to the U.S. Whether he does this privately with the Israeli leadership or makes the case openly; this contemplated attack by Israel must be averted, even if it takes Obama stating unequivocally that the U.S. would withhold all further financial and military support of Israel as well as the complete severing of relations between the U.S. and Israel.
For drastic actions (Israel contemplating a preemptive attack on Iran) would require equally drastic preemptive counter-measures by the only country capable of averting this impending disaster.
In his elaborate and insightful article in the September issue of the "The Atlantic" (available now on line) Jeffrey Goldberg outlines in detail the likely scenarios of Israel attacking Iran. In the synopsis of his article Goldberg writes, "A pre-emptive attack by Israel could be disastrous. It might happen anyway."
Goldberg's piece is replete with many interviews of Israeli leaders and their thinking. It should be required reading.
 "The Point of No Return", by Jeffrey Goldberg, "The Atlantic", September, 2010