Explainer 3 on the Iranian nuclear deal

Explainer Question 3: Why is this happening now?

Iranian President Hassan Rouhani was elected in 2013 on a campaign that promised to rebuild a lagging Iranian economy and repair relations with the West. That meant negotiating an end to the sanctions imposed by the P5+1 (the 5 permanent members of the UN Security Council; the US, UK, France, PR China, and Russia, plus Germany). The Obama Administration considers this to have been a sign that the sanctions regime worked--because of the pain we inflicted upon Iran, they were forced to come to the negotiating table.

After about a year and a half of talks, and countless signals from Israeli, American, and Iranian politicians that this deal just wasn't gonna happen, the P5+1 and Iran have come to a concrete accord by which Iran will dismantle some elements of its nuclear programme and permit an unprecedentedly thorough and expansive regime of inspections in exchange for the gradual lifting of sanctions and un-freezing of Iranian assets in the West. This was achieved despite plenty of loud, disruptive, antagonistic shouting by government figures outside of the negotiations in Iran, Israel, and the US.

Factors that have likely contributed to the talks finally resolving include:
Israel repeatedly showing that it does not give a damn about how the US feels about its actions toward Iran or the Palestinian people, and that it is not serious at all about a meangingful, negotiated peace with its Palestinian neighbours. This likely freed the Obama Administration of some of its reservations about pissing off the Israelis because, after all, so what? Israel won't respect us anyway.
The case is similar with Saudi Arabia, I reckon. What's Saudi Arabia gonna do if we piss them off? Effectively turn ISIS into a giant of jihadist terror? Oh, wait, they already did!

The sanctions coalition is fracturing. When the big round of sanctions was imposed, the US and Russia were operating as distinct yet cooperating counter-parties in world affairs. Russia is now taking on the US and the West as an enemy, and is happy to not cooperate with American foreign policy objectives much at all. The case is similar with PR China, whose relations with the US are increasingly confrontational. Moreover, given the threats on European hydrocarbon (natural gas and petroleum) supplies from Russian aggression, Europe is probably really keen on doing business with the Iranians again, as well.

The US and the Western security order really need Iran's help taking on ISIS, and they need ours. ISIS and al-Qaeda are common enemies of the West and of Iran.