In defiance of repeated warnings by the United States government, Turkey on Friday accepted a first shipment of parts of a Russian S-400 missile defense system. The US has been urging Turkey for months not to go through with the purchase, and has even put its planned sale of US F-35 fighter jets to Turkey on hold because it fears the use of a Russian missile-defense system by Turkey, a NATO ally, may undermine the security of F-35 fighter jets.

However, Turkey has refused to back down and even threatened to retaliate against US sanctions. By challenging the US on this matter, Erdogan is putting Turkey’s military alliance with the US and Europe at risk.

Read more: Turkey defies US as Russian S-400 missile defence arrives