By Jon Leyne
BBC News, Tehran
When President Vladimir Putin arrives in the Iranian capital he will be the first Kremlin leader to visit Tehran since Stalin sat down with Winston Churchill and Franklin D Roosevelt there in 1943.
Stalin (left) was the last Kremlin leader to visit Tehran
Iran would like to believe this visit will be just as historic as that summit during World War II.
"The mere fact of Putin's presence on Iranian soil is evidence that the West's policy of isolation is a failure and can be interpreted as a victory of Iranian diplomacy," the newspaper Iran News declared at the weekend.
But Iran and Russia have a difficult relationship in which nothing is easy to predict.
Whatever the shared interests between the two countries there is plenty of mutual suspicion, a fear on both sides of being used by the other.
"From the Iranian side I want to say that we want to work with Russia as a big power, as a neighbour, and as a country with some technologies that Iran needs," explained Abbas Maleki, chairman of the International Institute for Caspian Studies in Tehran.
"But at the same time Iran doesn't trust Russia as a country that can defend Iranian national interests."
RUSSIA AND IRAN
Russia building Iran's first big nuclear reactor, but project delayed
Russia blocking new sanctions against Iran
Iran rejected a Russian offer to supply nuclear fuel
This visit is all the more intriguing in the light of President Putin's deteriorating relationship with the West.
Will he play the "Iranian card" - taking Tehran's side in the dispute over Iran's nuclear programme?
There are two big tests.
At Bushehr, on Iran's Persian Gulf coast, Russian engineers are working to build Iran's first big nuclear reactor.
It is a programme that has been dogged by delays.
Russia claims Tehran is behind with its payments. Iran suspects Russia is dragging its feet for political reasons.
On Monday, the Iranian foreign ministry said there would be "good news" over Bushehr in the coming hours.
The good news Iran would like would be an assurance that the project will finally be finished next year.
Then there is the wider issue of Iran's nuclear programme.
Washington is pressing for a new round of UN sanctions because of Iran's refusal to end uranium enrichment.
Moscow has been blocking those new sanctions to enable the UN's nuclear watchdog, the IAEA, to work with Iran on clearing up outstanding issues.
Iran and the IAEA have an agreed timetable that continues till December to clear up those issues.
After that the dispute could well come to a head, and Russia will have to decide which side to support.
So far Russia has been sympathetic to the Iranians.
At a meeting with President Nicolas Sarkozy of France last week, President Putin said there was "no real data" to show that Iran was pursuing a nuclear weapons programme.
But President Putin is also reported to be frustrated with President Mahmoud Ahmadinejad, because of the Iranian leader's rejection of Russia's offer to help supply Iran's nuclear fuel.
Russia and Iran - the Islamic Republic and the country that not so long ago banned organised religion - are not easy allies.
Just before President Putin travelled to Iran, the Kremlin claimed it had discovered a plot to assassinate him in Tehran.
Officials began to cast doubt on whether the trip would go ahead at all.
It all sounded suspiciously like the Russian leader was having second thoughts, that he was considering bowing to Western pressure not to come here.
In the usually predictable world of diplomacy, the outcome of Mr Putin's visit here could also be up in the air until the very last minute.