Turkey is taking delivery of Russia's S 400 missile defence system. What this signifies is that Turkey and Russia are now "officially" allies. The first shipment of the S-400 landed in Ankara on July 12, according to Turkey's Ministry of Defense. (see image below)

Two more shipments are due, with the third delivery of "over 120 anti-aircraft missiles of various types… [scheduled] tentatively at the end of the summer, by sea." 

Reports confirm that the "Turkish S-400 operators will travel to Russia for training in July and August. About 20 Turkish servicemen underwent training at a Russian training center in May and June, …"(CNN, July 12, 2019)

How will the US respond?

In all likelihood, Erdogan's presidency will be the object of an attempted regime change, not to mention ongoing financial reprisals directed against the Turkish Lira as well as economic sanctions.