https://www.stratfor.com/analysis/geopolitics-iran-holding-center-mountain-fortress
According to this article, Iran is seen as a fortress due to the Zagros Mountains. I realize that being difficult to conquer does not mean impossible. But the article mentions the Mongols and Ottomans as two peoples that successfully invaded Persia. What distinguishes the Mongol and Ottoman invasions from invasions like Alexander the Great or Parthia? Did Persia's geographical location truly provide it with more protection compared to Mesopotamia?
The Zagros mountains only protect Iran from invasions from the west. The Mongols (as well as countless other groups throughout history - Parthians, Seljuks, Timurids, etc.) conquered Iran from the east. Iran has only been conquered from the west two times - by Alexander the Great, and by the Arabs in the seventh century. So yes, the Zagros mountains were definitely a formidable natural barrier. It's also wrong to say that the Ottomans successfully invaded Iran. They were able to temporarily hold bits of Iranian territory from time to time, but they never really conquered the country - not by a long shot.