Within the context of U.S strategic interests, why is the U.S so fervently opposed to Iran getting a nuclear weapon?

Question

It is understandable that Israel is opposed to Iran developing nuclear weapons given the comments made by Iranian leaders. However, how precisely does Iran developing nuclear weapons threaten American interests? I doubt, after all, that Iran would be able to develop ICBMs capable of targeting America.

One argument is that Israel is an ally and hence we should support it; however, if the sole threat Iran's nuclear program poses to the U.S is the threat it poses to Israel, why is the Israeli-U.S relationship so important strategically the U.S should go out of its way to prevent Iran from getting a nuclear weapon, with Obama indeed having affirmed in the past that "all options are on the table". Why exactly would a U.S president offer to risk the lives of American soldiers by threatening a preventative war on behalf of another nation?

In particular, what role, if any, does the Israel lobby play?

Answer 1

Iran is (fairly successfully) trying to be a regional hegemon - in a region which is vitally important to world economy, both due to oil resources AND maritime trade.

If it obtains nuclear capability, the strategic threat to USA isn't ICBMs (although, your lack of faith in Iran's ability to obtain ICBMs is somewhat laughable^[1]) - it is:

1. Ability to threaten the other countries in the region into submission - most notably, Saudi Arabia and the rest of its Sunni allies. If successful, Iran would control ~66% of OPEC's total oil reserves (>50% of world's) based on OPEC's own data.

2. Ability to supply nuclear devices to terrorist groups, since providing military support to terrorists is a standard strategy for Iran since 1980 (see tha latest events in Syria and even later, Yemen).

3. Ability to reduce United State's ability to project power in the region (if you force US bases out of ME countries and threaten US carriers with nuclear-tipped Silkworms or better antiship munitions, you basically deprive US of much of its ability to project force that way).

4. Nuclear exchange between Iran and Israel, assuming Israel does have nuclear capacity that everyone assumes they do. Obama may not grieve too much if a couple million Jews kick the bucket (or may be he does care, stranger things have happened) - but the economy of USA would surely feel a major kick if retaliation nuclear strike takes out a large portion of oil reserves and/or production.

As far as your laughable leading question about Israel lobby, you'll note that Obama met a lot more with Saudi Arabian leaders than Israeli ones on the topic. Call me skeptical, but I seriously doubt that Israel lobby is deeply concerned with Iran as a strategic threat to KSA. Nor are they privy to strategic retaliation plans by Israeli government that would affect #4 scenario.

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[1] - regarding ICBMs: they were successfully developed by 1950s USSR (devastated by WW2 with primitive science, technology and computing); and by North Korea (with no resources to speak of). And delivery to orbit was also developed by small private companies in 2000s as well. Contrast that to Iran, with a large and well developed economy (which will get a steroid boost now that Obama gave away many billions in sanctions); well developed industrial capacity and advanced science and engineering and deep human resource pool. And really, they don't even need to build from scratch. Russians (or North Koreans, or China) would gladly spread their ICBM wealth around in exchange for cold hard cash, not to mention an ability to stick some sand in USA's gears.

Answer 2

Meersheimer published a book, The Israeli Lobby and US Foreign Policy, in 2007. In the last chapter he covers Iran. He writes:

Iran's nuclear ambitions do not pose an existential threat to the US. If Washington could live with a nuclear Soviet Union, a nuclear China or even a nuclear North Korea then it can live with a nuclear Iran.

I think whilst Meersheimer is correct here, there is a great deal to be said for applying political pressure to prevent the proliferation of nuclear arsenals to countries which harbour nuclear ambitions, but do not have them. A little earlier, he writes:

Iran is widely seen as [Israels] most dangerous enemy as it's the adversary most likely to obtain nuclear weapons. Virtually all Israelis regard an Islamic country in the Middle East with nuclear weapons as an existential threat. The Israeli Defence minister, Binyamin Ben-Eliezer remarked one month before the Iraq war "Iraq is a problem ... But you should understand, if you ask me, today Iran is more dangerous than Iraq"

Sharon began publically pushing the US to confront Iran in Nov 2002, in a high profile interview in The Times (London). Describing Iran as the "centre of world terrorism" and bent on acquiring nuclear weapons, he declared that the Bush administration should put a strong arm on Iran "the day after" it conquered Iraq.

In late April 2003, Ha'aretz reported that the Israeli ambassador in Washington was now calling for regime change in Iran. The overthrow of Saddam, he noted, was "not enough". In his words, "America has to follow through. We still have great threats of that magnitude coming from Syria, coming from Iran.

I imagine he no longer views Syria as posing a threat. Meersheimer concludes his book by writing:

It is not surprising that Israel & it's American supporters want the US to deal with any and all threats to the security of Israel. If their efforts to shape US policy succeed, then Israels enemies get weakened or overthrown, Israel gets a free hand with the Palestinians, and the US does most of the fighting, dying, rebuilding & paying.

He asks the question: "can the Lobby's power be curtailed?" And answers this by saying:

this is not going to happen any time soon. AIPAC and it's allies (including Christian Zionists) have no serious opponents in the lobbying world. They know it had become more difficult today to make Israels case and they have responded by expanding their activities and their staff ... [moreover], major media outlets will remain sympathetic towards Israel no matter what it does.