Iran isn’t invadable because the US doesn’t have ENOUGH army.    From Wordnik.com.  [Think Progress » Harry Reid: No Good Military Options in Iran] Reference

Italy easily invadable and rather hard to defend.    From Wordnik.com.  [The Crest-Wave of Evolution A Course of Lectures in History, Given to the Graduates' Class in the Raja-Yoga College, Point Loma, in the College-Year 1918-19] Reference