The tiff with Turkiye over their purchasing of the S-400's had nothing to do with their being highly effective (they are). The issue that the US had was simply

The tiff with Turkiye over their purchasing of the S-400's had nothing to do with their being highly effective (they are). The issue that the US had was simply with a NATO ally purchasing Russian defense equipment. edit: respecting the Turks' rebrand, minus the special "u" character.