Because ww3 is around the corner and they dont want to be without any weapons. Nuclear weapons are a weapon but also a very good deterant
United States and Israel, and possibly other nations.
Iran
While Iran has the right to have a program to produce nuclear energy in accordance with IAEA guidelines and oversight. However, Iran has repeatedly prevented IAEA inspectors from visiting sites where nuclear weapons are rumored to be in production. As a result, the United States and most European states are very distrustful that Iran is engaging in a peaceful nuclear energy production program and believe that Iran intends to create nuclear weapons. The Western States also categorically opposes Iran having a nuclear weapon given that Iran has expressed militancy towards and supported/founded militant groups that act throughout the Middle East. If Iran embraces a more moderate regime and makes clear that they are willing to work within IAEA parameters, Iran should not have a problem with its nuclear program.
There are two questions here. Why does Iran need a nuclear bomb? -- Iran does not NEED a nuclear bomb. Iran WANTS a nuclear bomb since having a nuclear weapon would, in Iran's view, lessen the advantage that Israel has over its neighboring Islamic States. It would also be a symbol of Iranian power and a projection of Iranian influence and civilization. When will Iran have a nuclear bomb? -- The timeline for Iran gaining a nuclear weapon is unclear. It is likely that Iran will have sufficient fissile material to make a bomb by late 2015 or early 2016. However, the Israeli Mossad has performed a number of operations aimed at delaying the creation of such a weapon.
The actual answer may surprise even you: ID 1667019371. If Iran ever used a nuclear weapon on any country, let alone Israel, Iran would be destroyed.
No, there is not imminent threat of Iran building this past summer, a National Intelligence Estimate on Iran's nuclear capabilities was released showing that Iran was 10 years away from building a nuclear weapon. This estimate represents a consensus among the U.S. intelligence agencies. Nuclear war or atomic war is war in which nuclear weapons are use.
There are a lot of parameters to the deal, but at its most simple, the deal guarantees serious inspections of Iran's nuclear capabilities and effectively preventing Iran from getting a nuclear weapon in exchange for removing a number of sanctions imposed on Iran and unfreezing Iranian assets in foreign banks.
According to RT News as of 11/2013, Russia still supports Iran's nuclear program and is continuing to both broker a deal with their allies and provide support for the equipment they have already sold Iran. If I understand correctly, Russia sees cheap nuclear power as a good thing for Iran and doesn't believe that Iran has the intention to join the, "Nuclear Club".
The US has imposed trade restrictions against Iran in hope of pressuring their government to abandon its attempts to build nuclear weapons.
The US has no issue with Iran having a program to produce nuclear energy in accordance with IAEA guidelines and oversight. However, Iran has repeatedly prevented IAEA inspectors from visiting sites where nuclear weapons are rumored to be in production. As a result, the United States is very distrustful that Iran is engaging in a peaceful nuclear energy production program and most European nations share the US's skepticism on the matter. The United States also categorically opposes Iran having a nuclear weapon given that Iran has expressed militancy towards and supported/founded militant groups that act against a number of key US allies in the Middle East, like Israel, Saudi Arabia, and the Gulf States.Since I do not work for the State Department, this is the policy as far as I understand the State Department's articulations on the matter. As a result, take the answer with a grain of salt.
Iran's nuclear program is designed to facilitate (if not actually create) nuclear weapons. Additionally, the Iranian government is actively belligerent and continually declares both its enmity towards Israel and willingness to see that country destroyed. That combination makes other countries less interested in having a nuclear Iran.
Stuxnet