The dispute sparked by the Russian air defense system S-400 is the latest in a series of differences between Turkey and the United States, all centered on the different strategic and security visions between the two NATO member countries.
Washington saw the Russian system as a threat to its military equipment in Turkey, especially the F-35 stealth aircraft. Turkey has asserted that the deal is a sovereign right, and poses no threat to the US or NATO military presence on its territory.
The differences between the United States and Turkey in security and strategic issues have been decades old, and the two countries, which have successively the first and second largest NATO force, have tried to manage the dispute amid mutual skepticism.