The Turkish Ministry of Defense announced it received the first shipment of the S-400 system, although it is not yet fully in place or ready for use. For months, Washington urged Turkey to buy the American-made Patriot air defense system instead and has insisted that buying from Russia would result in economic and military penalties. Turkey has said it was not offered favorable terms on the Patriot.
Among the U.S. penalties would be cutting Turkey out of the multi-national F-35 production program, depriving the Turks of the sophisticated stealth aircraft and the economic benefit of helping to build them.
The U.S. concern is that the S-400 could be used to gather data on the capabilities of the F-35, and that the information could end up in Russian hands. But more than technology is at stake. Turkey has long been a key to the defense of NATO's southeastern flank, and some believe its willingness to buy key weaponry from Russia — long identified as NATO's main adversary — suggests the possibility that its alliance status is in jeopardy.
President Donald Trump recently expressed sympathy toward Turkey's decision to complete the Russia deal, although President Recep Tayyip Erdogan's government has been told that the S-400 is incompatible with NATO air defense systems and is seen by alliance officials as a threat to the F-35.
On Capitol Hill, Republicans and Democrats alike expressed dismay at the Turks' move.
"By accepting delivery of the S-400 from Russia, President Erdogan has chosen a perilous partnership with Putin at the expense of Turkey's security, economic prosperity and the integrity of the NATO alliance," the top members of the Senate committees on Foreign Relations and Armed Services said in a joint statement.
Russian President Vladimir Putin has long complained that NATO is designed to target Russia. Some see the Russian sale as an attempt to drive a wedge between NATO allies.
In their joint statement, the Senate committee leaders also called the Turkish action "a troubling signal of strategic alignment with Putin's Russia" and a threat to the F-35 program.
Derek Chollet, a senior defense official in the Obama administration, said Turkey's decision to begin taking delivery of the S-400 was not a surprise.
"It is a major problem for NATO - at best it will limit Turkey's role in the alliance, and at worst things could spin out of control," Chollet said. "Because our shared interests are so compelling I believe Turkey's place in the alliance will endure, but this will do lasting harm - starting with no F-35s, U.S. sanctions, and broader intelligence concerns."
Acting Defense Secretary Mark Esper said after Turkey's announcement that he was aware of the development and would discuss it later in the day with his counterpart, Hulusi Akar. The Pentagon and State Department did not immediately comment more directly on what actions the U.S. would take against Turkey.
U.S. officials have previously warned that sanctions would be imposed under the Countering America's Adversaries Through Sanctions Act if Turkey went ahead with the S-400 purchase. Sanctions would mark a new low in the already-tense relations between Turkey and the U.S. Last year the United States imposed sanctions on Turkey over its detention of an American pastor, triggering a Turkish currency crisis.
The prospect of a further rupture in Turkey's relations with Washington also raises a delicate issue rarely mentioned in public: the status of American nuclear weapons stored at Turkey's Incirlik air base. Turkey has had a nuclear role in NATO for decades, but this new split is likely to cause some in Washington to question the wisdom of keeping those nuclear bombs at Incirlik. Locations of U.S. nuclear weapons abroad are not publicly acknowledged by the U.S. as a matter of policy.
Turkey has refused to bow to U.S. pressure, insisting that choosing which defense equipment to purchase is a matter of national sovereignty.
There is a bipartisan consensus in Congress that Turkey should not be allowed to remain in the F-35 program if it refused to back out of the Russia deal.
Four senators — two Democrats and two Republicans — issued a joint statement Friday expressing disappointment that Turkey has chosen to buy a Russian-made system designed to "target and destroy" the F-35.
"Turkey is trying to play both sides, but we will not allow sensitive U.S. military technology in the F-35 to be at risk," the senators said. "Turkey cannot have both Russian and American defense equipment sitting side by side."