Despite it's history of military success and highly skilled military, Israel's armed forces are not comparable to the United States' in terms of military technology and in terms of quantity. As a result, from a purely operational standpoint, there will be a point in time at which Israel's military will no longer be able to strike at Iran's nuclear installations but at which the United States, given its superior technology, will be. If Israel waits for Iranian sanctions and international pressure to work and Iran advances and secures its nuclear program to a degree where Israel will not be able to launch a tactical strike like at Osirak in 1981 or in Syria in 2007. At that point, barring a catastrophic all out war scenario in which Israel must use its nuclear arsenal, Israel's defenses against the Iran's development of nuclear weaponry will rely on the United States military option.
While relying on the United States military umbrella might seem like not such a bad idea, it is a relatively untested and historically problematic concept. The United States most significant military intervention on Israel's behalf, during the 1973 Yom Kippur War, saw Henry Kissinger hold back aid from the Israelis in order to "let Israel bleed" so that Israel would give up more in post-war negotiations. Moreover, due to both implicit and explicit statements from the U.S., Golda Meir was unwilling to launch a pre-emptive strike against the Syrians and Egyptians even when Israel's military was caught by surprise but still had approximately 48 hours notice before the Egyptian/Syrian surprise attack. When Saddam Hussein launched SCUD missiles into Tel Aviv during the Coalition invasion of Iraq in 1991, President Bush applied extreme pressure on Yitzhak Shamir to ensure that Israeli airplanes did not attack Iraqi targets and risk alienating Arab members of the Coalition. Given this history of either non-involvement (in the Six Day War and War of Independence) and of mixed involvement (Yom Kippur War), President Obama is essentially telling Israel "trust me" to deal with Iran if push comes to shove.
Of course, there is a lot more to it than Obama's good will or belief that the U.S. can handle the situation better. Obama is creating a situation in which Israel sits under the Sword of Damocles and he controls when and if it will fall. By removing Israel's ability to unilaterally protect itself from Iran, Obama will hold significant leverage over Israel in its other affairs and relationship. Most likely, Obama will leverage his protection of Israel as a way to get Netanyahu or any subsequent Prime Minister to give greater concessions to the PLO so that Obama can become the president who brings "peace to the Middle East" and finally breaks Israel's intransigence in dealing with the Palestinians. By effectively removing Israel's control over its own security against Iran, the United States could force the Israelis to behave in a way that would enable Obama to promote his vision of the Middle East with far less regard to whether Israel thinks it best or not.
Now, considering the views of many in Obama's defense cabinet, there is no doubt that this will be a detriment to Israel. Robert Gates famously called Israel an "ungrateful ally" and Obama himself was caught calling Netanyahu a "liar" and a "headache to work with." Leon Panetta has repeatedly made public remarks that have undermined Israel's strategic position and has demanded that Israel make greater concessions in negotiations with the Palestinians. Of course, this raises a significant question of whether Israel should feel that it can trust the United States in general and the Obama Administration in particular.
There are, of course, factors that pull in both directions. While the United States vetoed the Palestinian attempt to get approval for statehood at the United Nations, Mahmoud Abbas' push for UN approval came about in large part because Obama's April 2010 statement to Mahmoud Abbas that he wanted to see a Palestinian state within 2 years. Emboldened by this, Abbas took the case to the UN, which caused Obama to backslide and demand renewed negotiations. Moreover, Obama's pressure on Israel to end Jewish residence in the West Bank as a precondition to negotiations with the Palestinians plays right into the Palestinian tactic of requiring Israel to concede everything the PLO wants as a condition to negotiating in the first place. While Obama has maintained a more balanced stance on certain other issues, a significant number of Obama's and his advisers' statements have either disparaged Israel or placed it at a strategic disadvantage.
All of this wouldn't be a major problem if Obama was not now turning around and telling the Israelis that he has their interests in mind if and when he decides to act on Iran. To believe that Obama has the interests of Israel foremost in his mind, even if Iran is a far more direct threat to Israel than it is to the U.S., is simply inconceivable. Despite popular belief and rhetoric to the contrary, the United States does not, and would not, act with Israel's primary interests in mind if it were deciding how hard a stance to take on Iran based on Iran's nuclear program hitting certain thresholds. Yet, at the same time, the U.S. is essentially refusing to allow Israel to handle the situation themselves at a time when they can still do so because to do that would (a) potentially implicate U.S. security guarantees to Israel (an idea which the Obama defense establishment abhors) and (b) would tarnish Obama's image as a peaceful man who is reaching out to the Islamic world. In fact, if the U.S. does eventually strike Iran, it will far more likely because the vast majority of Arab states are scared to death at the prospect of Iran becoming a dominant hegemon in the region and taking revenge for centuries of anti-Shia violence and repression. The last thing that the Saudis want to see is an Iranian government that can impose its will on the region.
The Israeli government is faced with a very troubling choice. On the one hand, it faces a growing risk that its own military will no longer be capable of destroying the Iranian nuclear program in a manner sufficient to set it back a meaningful amount of time and to deter future plans. On the other hand, the United States is applying significant pressure on the Israeli government to hand over control of its anti-Iranian options exclusively to the United States military. The decision to do so carries broad repercussions and gives the U.S. significant leverage and influence over Israel. There will come a day when America's willingness to act against Iran is dependent, for example, on Israel's willingness to compromise its strategic depth by withdrawing from the West Bank, or allowing Hamas free reign in Gaza, or evicting Jews from the West Bank and east Jerusalem. In this case, America and its divergent foreign policy goals, coupled with Israel's reliance on American military support to stop the Iranians, may prove to be as dangerous to Israel's long term vitality as Iran's military itself.
As the Czechoslovakians watched in horror as the British and French sold them out to appease the Germans by ceding the Sudetenland to the Nazis while completely disregarding France's agreed upon defense obligations, is it so difficult to believe that the Americans would sell out Israeli interests for America's benefit and appease Iran while demanding that Israel withdraw from certain territories to bring about peace in the region, even at is own expense? Just as the Czechoslovakians paid dearly for their faith in their larger, more powerful protectors against an extremely militant and growing regional threat, so too are the Israelis gravely mistaken if they believe that the United States will act in Israel's best interest when the day of reckoning with Iran comes.