For a year now, Ankara has been negotiating with Moscow to the S-400 surface-to-air missile defense system. Under the $2.5 billion agreement Ankara would receive two batteries of the antiaircraft missile from Moscow within the coming year and then produce two more batteries in Turkey. Turkey stepped up efforts to acquire its own missile-defense system after the US, Germany, and the Netherlands â€“ all NATO members â€“ decided at the end of 2015 not to renew their Patriot-missile deployments in southern Turkey. Spanish and Italian missile batteries remain in the country, but those systems are linked to the NATO air-defense system. Russia’s sale of S-400 to Turkey is making the US uncomfortable. Why?