Perhaps somewhat counter-intuitively, nuclear weapons are only useful for defensive purposes. To launch a nuclear device at another country is to commit your nation to oblivion. You might as well direct your attack at your own capital city, because before it even reaches it's destination, the retaliating missiles will have probably obliterated your entire country. Nuclear weapons are sought after, not for their offensive properties, but because they keep you from being bullied by other nuclear powers. They keep you free from the threat of invasion, because you can always threaten to launch if you see the enemy marching in over the horizon. This is why America, if they really believe Iran is developing nukes (and we know that America would never lie!), wants to stop Iran before they develop effective nuclear devices, because they know that if Iran becomes a nuclear power, the prospect of a grand invasion by US forces becomes an awful lot more tricky.
America is the only country to have ever used nuclear weapons on a civilian populous, and eventually wants to be the only country that holds nuclear arms. Look at what the USA are currently up to in Europe, regarding their Anti-Ballistic missile program. They are gradually moving in to surround Russia with their ABM missile defense shield, thus greatly reducing Russia's (hypothetical) ability to effectively use their nuclear arsenal. The USA proposes that these ABM bases are to defend America from "rogue-states" who may possess nuclear weapons, but what nuclear state, rogue or otherwise sits beside Poland, except Russia. Before the Berlin wall fell in 1989, Gorbachev was promised by the Americans that NATO would not move further east than Germany. Unsurprisingly, America lied, and we find ourselves in another US / Russian conflict, over a proposed ABM base in Poland. Russia has said that unless a new agreement is signed up, where Russia is involved in the shield, and America agrees not to use it against Russia, they will launch a military offensive to destroy these sites in Poland. Potentially, this is frightening stuff.
Money trumps all, even nuclear weapons. The Occupy movement has said that we are currently in a third world war, where the battles waged are economic and the weapon is debt. If you look at who America finds most antagonising today (Russia, China, etc.) they have nearly zero debt, and the countries they say are their closest "friends" (Europe et al.) are the ones that are drowning in it. Russia is joining the WTO in June of this year and hopes to build stronger economic ties with China, but using local currencies for trade rather than the US dollar. Thus, both countries will increasingly want less and less to do with US dollar as a reserve currency, weakening the US economy as it continues to dilute the dollar with quantitative easing.
Anyway, that's enough writing, it was only meant to be the picture of "Obama and Ahmadinejad" on it's own. Much like American hegemony, this post has sprawled into areas it has no business in.