A psychological state more than a geographic state of a person: a belief in a community's or culture's superiority, correctness, or specialness compared to other communities or cultures. Inspired by positive-minded well-meaning groupthink, increasing homogeniety over time, isolation-induced ignorance of other cultures or communities, fear of the unknown or being outnumbered (and a desire to compensate for their smallness amid the world), and lack of conflict with/lack of destruction by other communities (improving relative progress and social harmony and giving some creedence to their feelings of superiority).
Progressive and possessive attributes of English and Japanese histories show an island mentality.
(n.) The idea that you are better than everybody else, usually true.
My island mentality forces me to shit down the throats of every foreign riff raff that dares to offend. Consider yourselves warned, before you consider yourselves smoten!