The Iranian Foreign Minister Mohammad Javad Zarif arrives for lunch with the former Vice President of the European Commission at the Iranian Embassy during the 5+1 talks with Iran in Vienna on Nov. 18, 2014. Joe Klamar/AFP/Getty Images

VIENNA - Few events in history are momentous enough to reshape the world order. If the US and its partners are able to come to an agreement here with Iran to curtail its nuclear program, this week may be one of those times.

A deal has the potential to place the United States and Iran on the same side of the table, while testing America's longstanding - but already strained - alliances with Israel, Saudi Arabia, and countries in the Persian Gulf. It could also dangerously alter the power struggle in the region between Sunnis and Shiites.

That geopolitical landscape has already been muddied recently by other factors including the fight against terrorism, booming US energy production, plummeting oil prices, and frustration over Israeli settlement construction.

That's not to say the U.S. and Iran would become friends (or that it would shatter old alliances), but it could add to other areas of possible cooperation between Washington and Tehran like the fight against ISIS.

WHAT'S IN A DEAL?

That's also not to say a deal is a sure thing. It certainly won't be easy.

Talks opened today and the deadline is next Monday. Secretary of State John Kerry is expected to arrive later in the week to help close a deal.

The broad contours of any agreement have been known for some time. In short, the U.S. and its allies want Iran to significantly and irreversibly roll back its nuclear program so that it cannot build a nuclear weapon. They also want that program placed under ironclad monitoring. Crippling sanctions would, in turn, be lifted.

Yet big questions remain.

Would Iran be allowed any enrichment for energy production? What happens to the Iran's nuclear facilities? What about its nuclear material stockpile? Which sanctions are lifted, when, and in what order? Who would monitor implementation?

And just as important: can President Obama deliver on his end of the bargain?

The prospects of a deal got a boost recently when Russia said it was willing to take much of Iran's fissile material and return it in a form that can only be used in a nuclear power plant. That could solve the problems about the stockpile and enrichment.

Russia not only stands to make money off such an arrangement, but it also guarantees the influence it has coveted in the Middle East, helping preserve Moscow's seat at the table of world powers. But with Russia also suffering from depressed oil prices, could Moscow seek to delay a deal here to prevent Iranian oil, now under sanctions, from returning to the market?

Police guard in front of Palais Coburg where closed-door nuclear talks with Iran take place in Vienna, Austria, on Nov. 18, 2014. Ronald Zak/AP Photo

IS A DEAL LIKELY?

Ultimately, this may come down to whether two men - President Obama and Iran's Supreme Leader - are willing to trust each other.

U.S. officials are cautious and won't even say it's likely. That's perhaps to manage expectations, but also because it has been so hard to get this far and a negotiations have already blown through a previous deadline.

So far, there is no talk of another extension if talks fail this week. But the domestic political map is about to get more complicated both in US and Iran, so there may be a short window of opportunity.

Those looking to read tea leaves note that Secretary Kerry isn't here yet. Kerry had been scheduled to join talks at the start today, but officials say his schedule is up in the air and he remains in London. He's still expected to come here to seal a deal if things get close. If that's the case, as with last year's interim deal, Kerry's arrival could signal that an agreement is at hand.

WHAT ARE THE OBSTACLES?

These talks have been going on for a long time. It was in this same city back in 2006 that the United States joined negotiations started by three European countries - the UK, France, and Germany - and brought on board Russia and China, the other two countries wielding vetoes in the UN Security Council.

Since then the negotiations have been sporadic and halting. A breakthrough interim deal last November paved the way for this week's talks, but an initial deadline to reach a deal last summer came and went.

The half-steps and delays, celebrated by supporters as tiny signs of progress, have only made opponents of the deal more wary - and outspoken.

Israeli Prime Minister Benjamin Netanyahu has blasted the deal, Jewish lobbying groups, led by Aipac, have mobilized against it, and their powerful allies on Capitol Hill stand ready to block its implementation. They don't trust Iran and fear it will cheat.

While Obama could take some steps on his own, removing the toughest sanctions would require approval by Congress. The prospect of that happening became potentially more difficult with the Republican takeover of the Senate. Yet even within the president's own party he faces staunch opposition to anything but the strongest deal from lawmakers like Sen Robert Menendez, who remains the chairman of the Senate Foreign Relations Committee until the new Congress is sworn in next January.

Still, seeing as how it's not about to get any easier for either side, now may be President Obama's best - and last - chance for a deal for a long time.