That is the Legion of Merit - Commander Grade. The Legion of Merit was instituted early in the USA's involvement in WWII. Knowing that this was a "world war" and that we needed a way to award decorations to foreign leaders for providing assistance to us in the war effort, the Legion of Merit was instituted as an American version of the French Legion of Honour. Different grades were instituted so that the decoration was consistent-with/made-sense-to foreign militaries, who frequently award various degrees of the same medal, depending upon the rank of the person involved.
The first recipient of the award was a South American (possibly Brazil?) general who was assisting the US with trans-shipment of war material to the North African campaign. (I have a great picture at home out of a WWII National Geographic, but I don't have a scanner there, and I didn't bring the magazine with me here where I do have a scanner...)
Basically, the different award degrees are given according to the rank of the recipient. Heads of State get the highest degree, which is a large breast-star version of the medal, worn on the breast pocket of the uniform. Chiefs of Staff (such as the Turkish General in your photo), were awarded the next lower grade - Commander, which is the medal worn from a neck ribbon, just like our Medal of Honor. Lower-ranking Generals (than Chiefs of Staff) and sometimes military attaches of foreign govts serving here in the US, are awarded the "Officer" grade, which is a normal (as we Americans would think of it) medal, but with a large brass pin-miniature of the medal pinned to the ribbon.
The lowest grade is "Legionnaire" which everyone else gets. US personnel are eligible for award of this lowest-grade of the Legion of Merit. It is awarded for significantly important service, usually not combat-related. It would be the equivalent of the Silver Star medal, but for service, rather than combat. Interestingly, the LOM can be awarded with a "V" for valor device to US personnel, even though it's a service award.
Here is a link to Wikipedia's article on the LOM:
Legion of Merit - Wikipedia, the free encyclopedia It has a very good chart showing the degrees of the award, and there's a long list of recipients of the differing degrees. (Unfortunately, your man is not mentioned, although the Turkish President Celal Bayar, got the Chief Commander grade of the award the year prior to when this general was awarded the Commander grade.)
My guess is that the US was thanking the Turks for their help with our U2 flights, placement of Intermediate-Range Ballistic Missiles at the Soviets' doorstep, Joining NATO (which they did in 1952), opening Incirlik Air Base to the US, and other useful help provided against the Soviets at the beginning of the Cold War.