"the Russian S-300 missile air defense missile (seen on Wikipedia) is defensive, not offensive. Right?" - Maureen Ann Yes, it is defensive. And I applaud your seeking out information. The Wikipedia article on the S-300 family is actually pretty good. The specific system involved in the long delayed Iranian sale is the S-300PMU1. Your observation that it is 'defensive' is correct, but must be seen in a broader light. The classic weapon system, the Sword and Shield, succeeded for so long because of the interaction. The Sword is the offensive system and the Shield is the defense which protects the offensive system. Any increase in a nations defensive ability tends to create an improvement in it's offensive capability. Equally, a nation which lacks offensive capabilities has no ability to resist maneuver intended to circumnavigate any defense. Yes Maureen, the S-300 is defensive. What it defends may not be. The S-300 could defend an offensive capability as easily as an orphanage.
ISIS claims responsibility for bombing in Baghdad suburb; death toll hits 21 (Reuters)
from the article: Russia to supply Iran with S-300 defense systems