The Zagros Mountains protect Iran from would-be challengers. They also box in Iran. If Iran wants to project influence into Iraq, one of two conditions must exist. Either Iran must be incredibly wealthy and militarily strong, or it must be able to take advantage of weakness.

Persia was able to take advantage of weakness several times throughout history. But those times have always been when the area was weak and unable to resist Iran's advances.

Take, for example, the period before 2011. Iraq was Iran's biggest rival. Iraq was able to fight Iran to a draw in an incredibly bloody war from 1980 to 1988. This was despite having a population just one-fourth the size of Iran's. But the US invasion of Iraq in 2003 removed that threat.

Since then, Iran has been building an arc of Shiite influence in the Middle East. Syrian President Bashar Assad's Alawite regime was an ally of Iran. And Hezbollah's dominance of Lebanon meant Iran could project power all the way to the Mediterranean.

The Syrian civil war dashed that dream. The rise of the Islamic State was a threat to Iran. And a strong Sunni Arab power in the heart of the Middle East could not be tolerated. This has driven the US and Iran into an uneasy but stable partnership.

Iran wants to influence Iraq. On the other hand, Iran funds various Shiite militias in Iraq. This is to ensure Iraq's Sunni minority does not threaten the government in Baghdad.

The Assad regime has been weakened. Iran can no longer count on influence stretching through Syria to the Mediterranean.

The Arab world is in chaos. Israel is sitting pretty. And Turkey is a rising power. Turkey has almost the same number of people as Iran. It sits on more strategically advantageous territory, and it has a more advanced economy and military.

Turkey will be the dominant power in the Middle East. Iran will have to settle for being a regional power. It can influence countries in the region but not push beyond.