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It calls into question the decades-long strategic relationship between Turkey and the US, and even Turkey's credentials as a NATO member. It probably nullifies a massive contract for Turkey to buy US F-35 combat aircraft -- a plane the S-400 is designed to shoot down.
The deal also solidifies a deepening relationship between Turkey's President Recep Tayyip Erdogan and his Russian counterpart Vladimir Putin -- two leaders with little time for dissent at home and who need each other in Syria. And it provides the Turkish armed forces with an advanced weapon capable of covering most of Syria and their old adversary Greece (also a NATO member.)
The S-400 can shoot down aircraft at a distance of up to 150 miles (240 km) and intercept ballistic missiles up to 38 miles away.
In essence it is a destabilizing purchase in a region that could do without any more destabilizing. It is also an assertion by Turkey of its independence as a major regional power.
Tension between Turkey and US
Erdogan and other senior members of his ruling Justice and Development Party (AKP) have often played to anti-American sentiment among the party's conservative, nationalist base.
Erdogan was also infuriated by the US alliance with the Kurdish militia in Syria -- the YPG -- in the campaign to defeat ISIS. Turkey regards the YPG as a terrorist group affiliated with the PKK, which has fought the Turkish state for more than three decades.
The tension persists. The two sides can't agree on the establishment of a safe zone for refugees inside northern Syria. And this week, CNN reported that US military intelligence is observing a buildup of Turkish armored units that may be planning cross-border combat operations -- amid growing concerns that US troops operating in northern Syria will be caught in the middle.
"There are some indications" that Turkey is preparing for an "incursion" into Syria, but the intelligence is not yet definitive, one official said.
But all these difficulties pale in comparison to the fallout from the S-400 deal. Even before the first deliveries, the US warned that Turkey would be suspended from the F-35 combat jet program and stopped training its pilots.
Erdogan has said that excluding Turkey from the F-35 program would be "robbery," since Ankara has already invested more than $1 billion in the consortium building it. Altogether it planned to buy 116 planes.
US President Donald Trump has suggested the sanctions could be diluted, but many in Congress are determined Turkey should be penalized. According to a US federal law (the Countering American Adversaries Through Sanctions Act), the administration must levy at least five different penalties against Turkey. Just how punitive they will be is yet to be seen.
NATO is also concerned that the S-400 deal will affect Turkey's ability to cooperate with other alliance members. "Interoperability of our armed forces is fundamental to NATO for the conduct of our operations and missions," said one official.
Putin stirs up trouble
If Turkey wants to have any sway on the future shape of Syria, it has to engage with Russia. Buying the S-400 helped cement a necessary if not necessarily warm relationship.
Holding out an olive branch Friday, Turkish Defense Minister Hulusi Akar said Turkey was still considering buying US Patriot missiles "to cover our need for a long-range air and missile defense system." He also said the purchase of the S-400 "does not in any way mean a change of [Turkey's] strategic orientation."
But the course of Turkey's strategy seems set.
And you don't share your best jets with countries that are not your friends.