The Russia - Iran Allegiance
There is something nefarious about a Russian-Iranian axis to the Middle East region. Putin's Russia is making sure the world knows that Russia is viable and still potent adversary. It seized Crimea, took over a part of Ukraine incognito, buzzes NATO aircraft too closely, positions itself in a threatening manner at the Baltics. Now, after a delay of five years (largely because the US opposed the sale), Putin has approved the sale of 4 to 6 batteries of the ultra advanced S-300 defense system for $800 million. One reason is Russia needs the money but the other reason is to make Russian and Iranian cooperation more allied. Russia has many lucrative contracts with Iran for nuclear reactors and other infrastructure projects.
Iran is one of Russia's biggest customers. It seems Putin thinks it is safer to make Iran a friend now and then when they become nuclear Iran will be an ally. But Putin also despises the current US administration even more than before and the sudden sale reflects this. After the US placed economic sanctions on Russia for Crimea and the Ukraine, Putin's position on the sale changed.
The reason why the US does not want the sale of the weapon system is because it would make any military strike against Iran's nuclear facilities VERY difficult, if not , impossible. The SA-20 Gargoyle (NATO's name for the S-300) is so sophisticated that only stealth aircraft would be able to penetrate, and even those might have problems. The sale of this system changes everything and could make Iran's position in the nuclear talks more difficult depending how fast these arrive in Iran. Or, if there is a deal, should Iran cheat, make any airstrikes very hard to succeed.
The S-300 renders any F-16, F-18 airstrikes upon Iran useless. The only ones that could be successful are the B-2 (the US has only 20 of them), the F-22, and the F-35. This would make that any military action against Iran could come from the USA. Granted, the F-15, F-16 and F-18 could fly but could not stay very long within the S-300 zone. The sale will prevent Israel from even thinking of sending in a strike until they get a F-35. Even then, they most they can carry is a 2000 pound bomb, not strong enough to do permanent damage. Because the facilities are buried, the only aircraft that can carry a 30,000 pound bomb is the B-2. Once Iran has the S-300, which are mobile, any military strike will become very difficult. As experts stated, it is a game changer.
Long range cruise missiles could be used to destroy the S-300 once their location is electronically found, which is not hard, but as soon as the S-300 moves, the process become more difficult. Aircraft can jam and spoof the radar of the S-300 and then use long range missiles to target it. The missile system can hit targets flying 16-miles above the earth and in 2013, Russia was going to sell them to Syria but under US pressure did not. The S-300 is capable of setting up in under five minutes, tracking up to 100 incoming projectiles, and engaging sixor more targets simultaneously. It is designed to intercept aircraft, cruise missiles, and ballistic missiles, as well. When aircraft are within their detection range, there is a 80% chance they will be shot down. Depending on the variant, the detection range is between 90-200 km.
There is no doubt Iran will use them in around their nuclear weapon facilities and most likely be manned by Russian trainers for up to six months. All this comes into play with the nuclear negotiations. When delivered, Iran will be more difficult because they will feel empowered and feel there is nothing the West can do as they develop a bomb. They are not far from being right. If the US is reluctant now, with the S-300 there, even more so. This puts more pressure on the West to concede to Iran's demands. The only thing left are sanctions, which are effective. Since Iran has survived the sanctions, they might feel the price to get the bomb is worth it in the long term even if its population suffers. When the S-300 arrives is unknown, but if they are due by June or July, one can see why the Iranians are stalling for time.