According to the AP, an anonymous source sent in a diagram demonstrating that Iranian scientists are working on a nuclear bomb. For some reason, this is a boogeyman to the US and Israel.
From the US perspective, a single nuclear bomb in the hands of Iran is cause for contemplating war yet it is extremely unlikely that Iran has the capability of developing a ballistic nuclear weapon capable of striking the US. During the Cold War, both the US and Soviet Union built thousands of nuclear weapons for fear that not all would work.
From the Israeli perspective, Iron Dome has shown great efficacy in defeating Syrian ballistic missiles so it should prove minimally to be a deterrent and maximally a countermeasure to an attack.
Given both of these conditions, the build-up of a nuclear powered Iran seems excessive. North Kore has had a nuclear capability for a longer period of time has yet has yet to seriously takes steps towards attacking the United States. Pakistan and India, two diametrically opposed nations, both have nuclear weapons yet have not launched them at each other.
The other problem with the Iranian nuclear bomb scare is that it totally misses the real threat of a cyber attack. Stuxnet effectively attack Iran's power grid and nuclear facilities. The US faces a far greater threat should our banking systems of power grids be attacked, it would be far more devastating than even multiple nuclear weapons.
AP story here