The comparison with Soviet Union is simply wrong because whatever happened in Soviet Union was done by people of that union and not by daisy cutters and bunker buster bombs falling on hospitals and kindergartens. Iran is a capable nation. But even a capable nation can take so much. You see even if I bring you the best boxer in the world and put him to fight a average man, with instruction that the boxer is not to hit the man at all rather just defend himself, while the average man is instructed to beat the boxer as much as he wants, it is only a matter of time before the boxer will fall down. Right now Iranian military does not have much to retaliate with. Chemical and biological weapons are horrible but for US military they are jokes since they have all the gears to combat it. It is not like Iran Iraq war, when Iranians even did not have even Atropine vials, let alone enough masks and protection suits. Unless, Iran has nukes and ICBM's, US can win a war against Iran.
It took US 2 weeks to get Afghanistan, 3 weeks Iraq and 24 weeks Libya. In case of Iran since they are very capable it might take 40 weeks, a year max, even if Iranians try to cut off the oil and all sorts of stuff, Tehran will fall within a year. All those Shahab-3's also will make some causalities too, but at the end, they can do as much, even if Iran fires all of them numbering in tens of thousands hardly a city can be taken out completely. This is the truth. But if Iran has the nukes, then situation is completely different. Any aggression and mass of killings of Iranians will mean Iran can take the war to the city of the aggressors too.
Right now Iran can not do that. Only when an aggressor sees the potential victim to be capable of retaliation in kind, that it stops from raping the victim. The reason Iran has not been attacked till now, is not because Iran is so capable but because US has not yet decided if fighting a one year long war with Iran is really worth it. If some day it decides that or calculates that it is going to take less than one year, six months or whatever based on their army generals assessment, then US will come heavy. On that day, when US bombers are beating the Iranian schools and homes into potato mash, Iran will not be able to do anything about American cities. I encourage you to study about nuclear arms and how they prevent war. Start from
Mutual Assured Destruction and go from there to each countries
nuclear doctrine.
For these concepts you have to know that US military is designed around the idea of
Total warfare. It is not like how Iranians were fighting their war with Iraq. It is completely a different way of fighting. There is a reason why it is called total. It means the full force is brought on for total destruction of all things useful to the enemy. It is really horrible and once war starts US will not give up until Iran surrenders or become completely like Afghanistan. Iran has never fought such a war. So I encourage you to read about it.