The diplomatic talks were at times grueling. They came in fits and starts, and nearly collapsed more than once. But this evening in Geneva, the United States, Britain, China, France, Germany, and Russia reached a nuclear deal with Iran .

NBC News confirmed through multiple sources that a deal was reached, a historic breakthrough in the world’s decade-long nuclear standoff with Iran, and in the 35-year-long diplomatic freeze between Iran and the United States. […] Iran and six of the world’s powers … agreed on a “first step deal” that is meant to limit advancements in Iran’s nuclear program in exchange for easing some of the economic sanctions that have deeply hurt Iran’s economy. […] While the “first step” deal is currently set to last for a period of just six months, it has set off a massive sense of relief on all sides in Geneva as it is expected to make Iran less capable of building a nuclear bomb for the time being, while at the same time easing the financial pain Iran’s economy has been enduring under the sanctions. Perhaps most significantly, it also makes a final comprehensive nuclear agreement between Iran and the world suddenly more possible.

The specific details of the agreement, reached after marathon talks that concluded at 3 a.m. local time, have not yet been released. That said, the New York Times report sketched out the broad blueprint: The freeze [of Iran’s nuclear program] would last six months, with the aim of giving international negotiators time to pursue the far more challenging task of drafting a comprehensive accord that would ratchet back much of Iran’s nuclear program and ensure that it could be used only for peaceful purposes…. According to the accord, Iran would agree to stop enriching uranium beyond 5 percent. To make good on that pledge, Iran would dismantle the links between networks of centrifuges.”

The United States has reportedly agreed to ease sanctions by $6 billion to $7 billion, and the international community will have monitors in Iran with access to nuclear facilities, including the Natanz enrichment facility.

There are, to be sure, a series of hurdles ahead. What’s more, in terms of the politics, the agreement will likely be denounced by Iranian hardliners and U.S. conservatives who believe the Iraq war was a great idea and were outraged when the U.S. disarmed Syria of chemical weapons without firing a shot.

But the usual suspects notwithstanding, tonight’s breakthrough is nevertheless a landmark diplomatic achievement that seemed largely unthinkable up until quite recently.

President Obama is scheduled to address the nation very shortly. This post will be updated.

First Update : The U.S. State Department published this six-page fact sheet with more details on the agreement itself.

Second Update : MSNBC’s Andrea Mitchell described this as the most important diplomatic breakthrough between the West and Iran since the Iranian Revolution in 1979.

Third Update : Sen. John Cornyn (R-Texas), the #2 Republican in the U.S. Senate, responded to this evening’s historic diplomatic breakthrough by tweeting , “Amazing what WH will do to distract attention from O-care.” There’s something deeply pathological going on in contemporary Republican politics, and it’s just not healthy.