Other nations have obtained nuclear weapons and the world did not come to an end. China, India, Pakistan which is a Muslim country, and even North Korea which is part of George Bush's "Axis of Evil." So why is it a big deal if Iran gets them as well?
First of all, the world is not a safer place on account of Pakistan and North Korea possessing nuclear weapons. These countries haven't used them yet, but Korea is governed by a dictator with mental problems, and Pakistan's government isn't so secure.
Even though Pakistan is a Muslim nation, the government is run by the military. Normally we think of military governments as being bad, but actually it's a good thing in Muslim countries. Armies tend to be secular meritocracies and the generals are usually capable of rational decision making. Musharraf probably doesn't want to die in a nuclear exchange.
The problem Pakistan owning nuclear weapons isn't so much that the current government will use them, but that a future less rational government, run by religious zealots, might used them. The other problem is that nuclear technology seems to be leaking out of Pakistan, and out of North Korea as well. And that technology is leaking in the direction of Iran.
The more countries that have nuclear weapons, the more likely that one may eventually be used, and the more likely that terrorists may acquire one and set it off in the United States. The nuclearization fo the world's nations is a very bad trend. And Iran has already stated that they wish to help Sudan also become a nuclear power.
But Iran is especially worrisome because the government isn't run by rational people, but by religious zealots who believe that it's their mission to destroy Israel and destroy the United States who they call the Great Satan. It's one thing to legitimately worry about a country that says they only want to defend themselves. But for a country that publicly states over and over again that it wishes to destroy us, and who believe that dying in battle against us means ascension to Paradise, then we really ought to take these words at face value and not assume that they don't really mean it. Jewish people in Germany believed that Hitler wasn't serious about his anti-Semitic statements, but they found out that he was very serious indeed.
I am open to arguments why we should trust the Ayatollahs of Iran more than we should trust Hitler. Please try to convince me otherwise.
Here is my sobering list of possiblities for what happens when Iran obtains nuclear weapons:
(1) Launch first strike against Israel. Iran has already stated that they think Israel should be wiped off the map. The benefit of this tactic is that it's a sure thing to hit Israel with a ballistic missile. The drawback, from Iran's perspective, is that Israel has submarines with nuclear warheads so will surely launch a counterstike that destroys the country. But this may be considered acceptable losses to religious fanatics. 25% probablity.
(2) Give a nuclear bomb to terrorists who will bring it into the United States and detonate it in a populated city such as New York or Washington DC (for symbolic purposes). This will surely punish us good for being the Great Satan. The benefit of this tactic is that Iran might be able to maintain plausible deniability and thus avoid a nuclear counterattack. But the problem here is that it's not so easy to smuggle a bomb across the world, and it may be intercepted U.S. counterintelligence. If Iran only has enough enriched uranium to build a single bomb, and the goal is to actually detonate it, trying to smuggle it into the U.S. is a risky strategy. 10% probability.
(3) Iran invades southern Iraq (after we've withdrawn) in order to unite the Shiites, and then invades Kuwait as well because they have a lot of oil and Saddam has already proven that Kuwait is an easy target. Iran will say that this is to punish Kuwait for cooperating with the Great Satan. Then Iran will brandish its nuclear weapons and threaten to use them if attacked. 35% probability of this or a similar miltary excursion.
(4) Iran peacefully does nothing with its nuclear weapons. 30% probability.
Given the high probability that very bad things will happen with Iran obtains nuclear weapons, it is my belief that we shouldn't allow it to happen.