The charge that Iran is engaged in a program of nuclear weapons development is based on a number of claims that are very well known to be false by the very US government officials making them. I'll analyze five of the most common charges below.
Charge: The idea that Iran's nuclear program is for electrical power production is nonsensical on its face. Iran has huge reserves of oil and gas to use for power production.
--- Answer: This charge is based on an elementary error in economics. If I have an apple tree and I can sell the apples for $1 each, then any apple that I eat myself costs me $1, even though I never open my wallet. Iran is a poor country with a large population that has tripled since 1970 and is still growing rapidly. It needs money for health care, education, roads, police, defense, etc. Every barrel of oil they use for power generation is one they cannot sell to pay for other things. With present and future oil prices, countries all over the world are finding that nuclear energy makes economic sense. It makes even more sense in Iran, which has large reserves of natural uranium.
Charge: Even if the Iranians are interested in power generation, they don't need to develop the uranium enrichment process. The US and Europe have offered to supply Iran with enriched fuel for its reactors.
--- Answer: Iran's known uranium reserves can produce as much electricity as 45 billion barrels of oil - half as much as its total oil reserves. To ship this uranium out of the country to be processed would make no financial sense. Also, there is some history to this question. In the past, the Shah of Iran signed contracts with the US, France, and Germany and paid up front for large amounts of nuclear fuel to be delivered later. After the Shah's government fell, under US pressure, none of the contracts were fulfilled and neither was the Iranian money refunded. Iran intends eventually to generate most of its electricity by nuclear energy. If the US controls its nuclear fuel, Iran could find its economy devastated at the whim of any future American President. They couldn't possibly be expected to put themselves into such a completely vulnerable position.
Charge: By secretly building the enrichment facility at Natanz, Iran violated the rules of the NPT (NonProliferation Treaty) and therefore cannot be trusted, even if they follow the rules in the future.
--- Answer: The NPT only requires that a signatory reveal and allow inspection of any new facility 90 days prior to actually beginning uranium enrichment there. In February 2003, when President Khatami revealed the facility at Natanz and invited the IAEA (International Atomic Energy Agency) to inspect it, Iran hadn't begun any processing there and was thus in compliance with the actual conditions of the NPT. It should be noted that Iran was one of the first countries to sign the NPT when it was proposed in 1968 and that the three countries in their neighborhood that didn't sign and went on to develop nuclear weapons, namely Pakistan, India, and Israel, are considered close allies of the US in the "War on Terror."
Charge: When the Iranians finally invited the IAEA to inspect the once-secret plant, the inspectors detected traces of highly-enriched uranium on the equipment, indicating that the Iranians had been producing weapons-grade nuclear fuel there.
--- Answer: After four years of hard work the Iranians have just succeeded in enriching uranium to about the 3% level at which it is useful for power plant fuel. Highly-enriched uranium, usable in weapons, is enriched to about the 90% level, something the Iranians could not possibly have accomplished at that time. Most of the equipment came from Pakistan, where weapons-grade enrichment has been done for years. Everyone knew perfectly well that that was where the traces of highly-enriched uranium came from, but still the Bush administration managed to create a huge commotion over this completely fake issue. The recent IAEA report agrees that the issue of those traces has been satisfactorily resolved.
Charge: Iran has shown a pattern of noncooperation and has refused to answer questions about its past nuclear activities.
--- Answer: The best answer to this charge is to quote from the recent Understanding Between the IAEA and the Government of Iran. (See full text at: payvand.com/news/07/aug/1222.html) "These modalities cover all remaining issues and the IAEA confirmed that there are no other remaining issues and ambiguities regarding Iran's past nuclear program and activities. . . . the agreement on the above issues shall further promote the efficiency of the implementation of safeguards in Iran and its ability to conclude the exclusive peaceful nature of the Iran's nuclear activities."
--- In general, the whole Iranian nuclear weapons issue has been conjured up as justification for an attack on Iran.