The Problem: If Turkey and Iran have a direct war rather than just a proxy one like now

Right now, the war in Syria between Sunni Rebels and Assad as well as the war in Iraq between Iraq armies (which are all Shia) against ISIS, Turkey has to some degree supported Sunni fighters (which are ISIS in Iraq) and all Sunni Rebel Fighters in Syria because Turkey is primarily Sunni Muslim.

So, now ISIS is being defeated and Erdogan has had a coup so he can make the country a Sunni Islamic Caliphate, you are going to see Sunni Muslim men from around the world, coming to Turkey to join the Turkish Army to fight Iran and all Shia Muslims to the death.

This is pretty logical if you are a Sunni Muslim.

However, here is the real problem here.

Traditionally, (especially since Iran too over 400 U.S. Embassy workers around 1979, the U.S. has been enemies with Iran. Another fact here, The Soviet Union and now Russia is allies with all Shia Muslims in IRan and Syria.

The U.S. position is more complicated because we helped install a Shia Government in IRaq because in Iran and IRaq the majority of people there are Shia Muslim. So, because Americans as a democracy believe in Majority rule Iraq became a Shia Muslim ruled country through the United States. And when Saddam Hussein was killed this helped Iran with it's plans to "Take over" the entire Middle East and to kill as Many Sunni Muslims as possible there. Because Shia Muslims are only around 300 million but Sunni Muslims are around 1.1 billion right now.

The other problem I see is in a war like this between Turkey and Iran it would be difficult for Turkey to win this war without the U.S. or Turkey using nuclear weapons.

Iran could also not win this war unless Russia or Iran used nuclear weapons.

The other problem with this war is if Turkey is still in NATO when this war begins in earnest then NATO might be legally required to aid Turkey in it's fight with IRan.

HOwever, now that Erdogan is becoming more and more a dictator and democracy is over in Turkey, war with Iran is inevitable going from the proxy wars in Syria and IRaq to a real all out war between Turkey, Jordan, Saudi Arabia and many other Sunni states fighting IRan. But, like I said neither side can actually win this war without nuclear weapons used by someone on either side.