Silly Dan. The US and Israel would never have made such a puerile threat. Obviously the US and Israel and Chinese and so on will seek to defeat defense systems and seek to design new defense systems to overcome attackers own developments that may threaten to overcome old defense measures ad infinitum. Its a given and is a continual process since the beginning of warfare. Not a case of 'threatening to design a countermeasure unless they refuse to supply Iran' !! Frankly that is incredible. You obviously have no idea what has or has not been said. Though it seems pretty likely that Israel and the US are none too pleased with the prospect of the S 300 sale going through. What even a fellow like you should be able to figure out, is that if the US or Israel have a worthwhile measure to defeat or disable the S-300, or can easily design one in quick fashion, they will not be overly concerned at the prospect of Russia supplying such a system to Iran. Would they? Ask this question. Are they concerned or are they pretending to be concerned in order to lull the Iranians into a sense of false security? If you accept the former is true and they are concerned then your claim is nonsense. If you think the Israelis are bluffing the Russians and therefore the Iranians too, then you read too many novels.
U.S. defense contractor accused of copying secret military documents (Reuters)
from the article: Russia to supply Iran with S-300 defense systems