In October 2017, a diplomatic dispute between the USA and Turkey resulted in each country restricting the issuance of entry visas for the other country: most American travelers could not obtain a visa to visit Turkey, and most Turkish travelers could not obtain a visa to visit the USA.
Some travelers using US passports have reported that they were able to obtain Turkish visas at the Istanbul airport if their passport showed that they had not arrived directly from the USA, but had spent at least 24 hours in another country before flying to Turkey.
This is unofficial, unconfirmed information! Only the Turkish government can determine and apply rules for visa issuance. These rules may or may not be publicly available; they may change at any time without notice; and they may be applied differently at different times, in different places, by different government officers.
In short, if you travel to Turkey after having spent at least 24 hours in a country other than the USA, you cannot be assured that you will be able to obtain an entry visa upon arrival in Turkey. You may be required to depart Turkey immediately.
Those of us who love travel, and love visiting Turkey, hope that this diplomatic dispute will be resolved quickly, and visa restrictions removed, so that normal travel conditions may exist for both Turkish and American travelers.
—by Tom Brosnahan