It has the potential of transforming the Middle East.
Or it could go down in history as one of the most monumental failures of modern diplomacy.
With the nuclear accord between Teheran and world powers now in force, the verdict on this pact hangs in the balance.
What is clear is the deal has intensified the clash between competing visions of Iran's future, all of which will play out during 2016. In February, Iran will elect a new Parliament and Assembly of Experts, the body that will pick the next supreme leader, with the current leader Ayatollah Ali Khamenei aged and in failing health.
The coming year will also be awkward for US President Barack Obama. Most of the attention in the Middle East is now diverted to the wars in Syria and Iraq, and the need to defeat the terrorist group Islamic State in Iraq and Syria.
But Mr Obama's broader objective is to anchor Iran as a factor of stability in the Middle East, a difficult task just as he embarks on his last year in office, and is therefore in his least influential period.
Both Mr Obama and Iranian President Hassan Rouhani will continue to act prudently, attempting to reverse decades of suspicion and hostility by beefing up the nuclear accord with a variety of discreet dialogue channels.
But the omens are not good for such a rapprochement, and the decision not to submit the deal to congressional ratification may be Mr Obama's gravest error: since it is not a treaty, it can simply be discarded by any leader who next occupies the White House.