Excellent post that is the reason that people sholud carry. If a guy is standing in an elevator waiting for it to fill up and kill everyone on it and all the sudden on the 8th floor 3 guys with weapons strapped to there hip steps on he might re think his position.
the reason countries don't come here and take us on very often is because we have a bunch of bad *** **** and were are not scared to use it. Why do you think we have not kicked the **** out of iran yet? If the had no weapons to amount to **** do you think we would still just be sending Emails? They have nuclear capabilities and we are scared of that and because of it we are for the most part leaving them alone.
In a way, you're right. The danger poised by guns is so significant that carrying one yourself may actually be a logical response to it at an individual level. However, as a matter of public policy, saying..."guns are really dangerous and lots of people are getting killed. We need more guns" is nonsensical.
Glad you brought up Iran (which, by the way, does not have nuclear weapons according to every report I've ever seen.) Iran is acting rationally in pursuing nuclear weapons..they would, as you pointed out, dramatically increase the costs associated with any invasion of Iran. There is another side to this however. Were a country to invade a nuclear Iran, they would have no choice but to use massively overwhelming (read: nuclear) force. So while a nuclear Iran may be less likely to be invaded, the consequences of an invasion would be much more likely to be total destruction.
Even if one decides nukes are good for Iran, it does not follow that nukes are good for the world. The more countries have nuclear weapons, the greater the number of conflicts that could potentially trigger a nuclear world war. Individually, countries may be less likely to be invaded, but collectively there is a greater likelihood of complete destruction. Mutually assured destruction works great right up to the point where it doesn't work at all. The start of WW1 is, perhaps, the best example of what I'm talking about.