The presidents of Russia and Iran will meet in the Azerbaijani capital, Baku, tomorrow, hoping to mend an unprecedented breakdown in relations between the two traditional allies.
The Caspian Sea Summit of leaders of five littoral states seeks to address overlapping claims that the region's powers have to the vast energy riches believed to be buried offshore.
But the encounter in Baku between the Russian president, Dmitry Medvedev, and his Iranian counterpart, Mahmoud Ahmadinejad, is sure to overshadow a summit that has a history of ending with little progress.
Dmitry Trenin of the Carnegie Moscow Centre said: "This represents Iran's last chance to come to terms with the international community."
Rajab Safarov of the Contemporary Iranian Studies Centre in Moscow said: "It is an extremely important meeting that comes amid an unprecedented deterioration of the bilateral relations. Our relations have not been this low at any point in our recent history." he noted.
Russian-Iranian tensions came to the boil this September when Moscow, after repeated delays, officially dropped plans to supply Tehran with high-precision S-300 missiles and a batch of other sensitive arms.
The S-300s are seen as Tehran's best defence from any potential attack by Washington and were the subject of vociferous objections from both the US administration and Israel.
Iran did little to hide its displeasure with Russia's reversal.
Mr Ahmadinejad this month accused Russia of falling "under the influence of Satan [the United States]" and selling out "to our enemies."
Iran has since said that it will test its own homemade S-300. But analysts question the country's ability to quickly recover from the Russian blow.
The S-300 saga in fact only manifests a slow creep by Moscow away from its unquestioning support for Tehran.
Russia has already backed a series of United Nations sanctions resolutions against the Islamic nation and Mr Medvedev himself has voiced a growing sense of frustration with the persistent nuclear ambitions of Tehran.
Mr Medvedev broke with years of Kremlin tradition by admitting this summer that Tehran was "nearing the possession of the potential" to build a nuclear weapon.
The pronouncement led to another round of name calling - Mr Ahmadinejad labeled Mr Medvedev a "mouthpiece" of Iranian enemies - and a recognition of a shift in Moscow's stance from Washington.
Analysts say that Russia was particularly angered by Iran's decision last year to reject a compromise deal under which it would have processed most of the Islamic state's stockpile of enriched uranium.
It was turned down after some indecision by Tehran in what analysts see as a turning point in its relations with the Kremlin.
Viktor Kremenyuk, deputy director of the USA-Canada Institute, said: "Iran tried to play games over the deal and received a firm rebuke. We took away their S-300s and began supporting the US position at the UN Security Council."
The Caspian summit itself, the third gathering of nations that also includes Azerbaijan and the Central Asian states of Kazakhstan and Turkmenistan, was unlikely to make much progress on its most important dispute: how to split up the sea.
Iran insists on dividing the Caspian into five equal portions, while the Azerbaijanis are angling for access that corresponds to each country's coastline.
The question deals with much more than sovereignty issues: it determines which nation gets access to the juiciest part of the energy pie.
Moscow is willing to accept some sort of compromise but even Russian sources conceded that this was unlikely to happen in Baku.
The Voice of Russia state radio service concluded on its website: "It cannot be excluded that this summit will end up accomplishing nothing because Tehran is unwilling to accept any compromise."