It questions the decade-long strategic relationship between Turkey and the US, and even questions Turkey's qualification as a NATO member. It is likely to lift a massive deal for Turkey to buy US F-35 fighter aircraft – an aircraft targeted for launching the S-400.
The treaty also consolidates a deeper relationship between Turkish 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 its old opponent Greece (also a NATO member).
The S-400 can shoot down aircraft from a distance of up to 240 miles and catch ballistic missiles that are up to 38 miles away.
Essentially, it is a destabilizing purchase in a region that could do without further destabilization. It is also a statement by Turkey about its independence as a regional superpower.
Tensions between Turkey and the USA
Erdogan and other high-ranking members of his ruling Justice and Development Party (AKP) have often acted against American sentiment among the Party's conservative nationalists.
Erdogan was also angry at the US alliance with the Kurdish militia in Syria – the YPG – in the fight against ISIS. Turkey regards the YPG as a PKK terrorist group that has been fighting the Turkish state for more than three decades.
The tension remains. The two sides can not agree on establishing a safe refugee zone in northern Syria. And this week, CNN reported that US intelligence is monitoring a build-up of Turkish tank units planning to conduct cross-border combat operations in the face of growing concern that US troops operating in northern Syria will be caught in the middle.
There are indications that Turkey is preparing for an "invasion" into Syria, but the intelligence service is not yet final, an official said.
But all these difficulties pale in comparison to the consequences of the S-400 deal. Even before the first deliveries, the US warned that Turkey would be excluded from the F-35 fighter program and stopped training its pilots.
Erdogan said Turkey's exclusion from the F-35 program would be a "robbery" as Ankara has already invested more than $ 1 billion in the consortium that is building Turkey. Overall, she planned the purchase of 116 aircraft.
US President Donald Trump has proposed diluting the sanctions, but many in Congress are determined to punish Turkey. Under a US federal law (Countering American Adversaries Through Sanctions Act), the government must impose at least five different sanctions on Turkey. How punitive they will be remains to be seen.
NATO is also concerned that the S-400 deal will affect Turkey's ability to work with other Allies. "The interoperability of our armed forces is fundamental to NATO in carrying out our operations and missions," said one official.
Putin makes trouble
If Turkey wants to influence the future form of Syria it has to deal with Russia. The purchase of the S-400 has helped to consolidate a necessary, if not necessarily warm, relationship.
Turkish Defense Minister Hulusi Akar held up an olive branch on Friday, saying that Turkey is still considering buying US Patriot missiles "to meet our demand for air – and long-range missile defense system ". He also said that the purchase of the S-400 "in no way implies a change in the [Turkey’s] strategic direction."
But the course of the Turkish strategy seems to be fixed.
And you do not share your best jets with countries that are not your friends.