Is this for being concerned about Turkey's self-defense capabilities or is it more about that Russia competes with the U.S on arm deals?
What reasons does the US havefor being concerned about Turkey's S-400 deal?
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The BBC has an article on Turkey's purchase and it contains a section on US concerns. This follows from the fact that NATO and Russia aren't on friendly terms.
From the BBC article:
So why is Washington so concerned about Turkey buying the S-400?
For a start it is almost unprecedented for a Nato country to buy such a sophisticated piece of Russian military hardware. Greece operates the Russian S-300 but these it obtained indirectly. They were purchased by Cyprus and transferred to Greece after Turkish objections.
The S-400 system is highly sophisticated; this is an area in which Russian technology is impressive. But there are practical problems too.
According to an article by Reuters citing US officials, the purchase of Russian jets may be because those don't have protections against firing on NATO jets. The US officials speculated to this being a factor in Turkey's decision making in relation to the coup attempt in 2016. Quoting from the Reuters article (more quotes from officials in the article):
While they recognize that worsening U.S. relations with Turkey in recent years and Russia’s growing clout with Ankara may also have helped sway Turkey toward buying the S-400 system, three U.S. officials and a defense source told Reuters of another working theory that has gained ground within the Trump administration.
One reason Erdogan may have chosen to buy from Russia rather than from a fellow NATO member is that he might be wary of his own air force, which played a major role in the coup attempt on July 15, 2016, these officials say.
S-400 missiles, which Turkey began taking delivery of last Friday, would be better at fending off any attack on the Turkish government from its own jets than a U.S.-supplied Patriot system would, experts say.
Raytheon Co’s’s Patriot missiles, which have been on offer to Turkey, would have safeguards to help avoid “friendly fire” against other NATO warplanes, such as Turkish air force jets.
“You have to ask yourself: Why would Erdogan really want a Russian system?” one of the U.S. officials asked. “He doesn’t trust his air force.”