On October 5, 2017, Saudi Arabia signed a Memorandum of Understanding (MoU) on the purchase of S-400 air defense systems from Russia. The MoU was signed on the sidelines of a visit by Saudi King Salman to Moscow on October 4-5. Saudi Arabia also signed a Memorandum of Understanding with Russia to help the Kingdom in its efforts to develop its own military industries.
In an interesting move, on October 6, just a day after Russia agreed to sell air defence systems to Saudi Arabia, the US State Department also cleared a potential $15 billion deal for Saudi Arabia to purchase the Terminal High Altitude Area Defense (THAAD) missile defense system. The timing of the approval of THAAD systems by the US is of particular interest. Saudi Arabia has traditionally been closer to the US. This was the first time a Saudi King has ever visited Moscow. The US would not be happy that Russia is winning influence with its ally, as well as the fact that the two signed a defense deal worth $3 billion.