That’s the question being asked regarding Turkey’s decision to buy—and subsequently to take delivery on—Russia’s AS-400 anti-aircraft missile system and the impact that has, or should have, on our alliance with Turkey.
The question, though, assumes we have an alliance. Formally, one exists, but it’s in name only outside of NATO (and with Turkey cozying up to Russia, that one is in flux, too); Turkey has chosen functionally to walk away from any bilateral arrangement.
I think, though, the decision to cancel Turkey’s F-35 buy is sufficient. We don’t need to apply sanctions; we don’t need to have much of anything to do with Turkey, good or bad, outside our NATO obligations.
Turkey just isn’t that important to us. It’s a convenient path into the Middle East, but even that is questionable. Turkey, after all, refused to let us operate from there during the second invasion of Iraq, in contrast with Saudi Arabia’s and Kuwait’s active support. That refusal complicated military matters and led to a longer fight than necessary, with higher casualties all around.