In 2007, Russia signed a $800m deal to supply Iran with the S-300 missile system.
The news was trumpeted with much fanfare in both Tehran and Moscow.
It was a big trade deal for Russian President Vladimir Putin's government and a big security boost for Iran.
But it was never delivered. Increasing Western sanctions on Iran meant Russia shelved the deal.
Then this week the Russians backtracked and lifted the embargo.
Immediately Israel was up in arms. Israeli Prime Minister Benjamin Netanyahu, according to the Kremlin, called Putin to voice his concerns and the Russian leader responded by saying that the weapons were purely defensive, and did not pose a threat to Israeli security.
That was not enough for the Israeli leader. Israel is doing everything it can to derail a deal on Iran's nuclear programme that could be signed by June.
Iran wants the missile defence system badly. It is an anti-aircraft system that can hit aircraft and even ballistic missiles of up to 150 KM away.
That will prove a significant challenge to any air attack, and given Israel's open warning that it might hit Iranian nuclear targets is a concern to Tehran, however military experts say that the Israeli capability to hit targets inside the country has been overstated.
Russia and Israel have also have a complicated relationship and Israel has in the past sold arms to Russia's enemies. In Georgia in 2008 Israeli made weapons proved a challenge to Russian forces.
At that time it was noticed by Israeli commentators, who warned that the Russians were supplying Iran and Syria with weapons and Moscow did not need an excuse to sell those countries more advanced weapons.
More recently Ukraine, another Russian enemy, has received weapons from Israel, but under a tacit agreement it stopped supply in what many consider an unspoken de facto deal.
If Russia does not supply Israel's enemies, then Israel will not supply Russia's.
But with the lifting of the ban on the S-300, Israel faces a stark choice. Does it risk the wrath of Russia by supplying weapons to Moscow's enemies, or does it remain silent and figure out a way to beat Iranian air defence systems?
A clue maybe be found in Istanbul.
Quietly Israel were allegedly training Turkish fighter pilots, which began in 1997 according to reports from the time, advanced skills to beat air defence systems and have taken a very close look at the S-300 which the the Greeks had been using and was a concern for Istanbul.
By training the pilots Israel got a good look at the capabilities of a deployed S-300, knowledge which for the Israelis was very useful.
So beating the S-300 is one option. But what else can Israel do stop any further expansion of the Russian-Iran relationship?
Hardline diplomacy in at the United Nations and the EU is well traveled path for the Israelis and its likely they will use all the back channels they can to quietly put pressure on Russia. What Israel is truly concerned about is Iran's re-entry in the world community.
The sale of weaponry, lifting of sanctions, any nuclear deal all bring Iran back into the international fold and that is not something Israel wants.
Netanyahu has been clear repeatedly saying that any influx of money Iran receives after a deal has been struck will further finance its regional ambitions, saying "it will receive billions of dollars to finance its war and terrorism machines, with international legitimacy".
There is a lot of talk in the Israeli press about what Iran might do with the S-300.
Fears it could be transferred to Hezbollah in Lebanon, into Syria to back president Bashar Al Assad's regime are key Israeli concerns, but once again this might be political scare mongering than actual fact.
Iran has key strategic sites it needs to defend, risking those to put the S-300 in a place where Israel would have to respond with force like Lebanon or Syria makes no strategic sense. Also the numbers of S-300 being sold to Iran, according to reports, are quite small.
It might only be five separate missile platforms which Iran would have to deploy selectively.
From a purely security perspective Israel shouldn't have an issue with Russian arms sales to Iran, as the S-300 is a defensive system.
Politically though it provides a excuse to Israel to ramp up its anti Iranian campaign and keep up the pressure on Tehran. Russia has other concerns.
With a deal between Iran and the West in the being negotiated, Russia is looking to secure its influence in the region, and this might be what is driving Russian policy, and that is why it might be easing up on Iran and looking to keep it as partner.
Russia has plenty of trade interests in the country, including nuclear power plants, heavy industry and the like and maintaining and expanding that relationship is key part of Russia's foreign policy.
In many ways the S-300 is not about security or defence but a symbolic show of faith in Iran and one that has Israel worried.
Israel has not responded publicly with a concrete move yet, but it will have to and given the tone coming out of Israel, it is only real choice may well be to risk the wrath of Russia and supply arms to Moscow’s enemies.
Source: Al Jazeera