Russia has begun deliveries of its S-400 anti-aircraft missile system to China in accordance with a 2014 contract, The Diplomat reported Jan. 22. The defensive missile systems follow another recent Russian delivery to China of ten Sukhoi Su-35 fighter planes, as well as a Financial Times report that a Chinese company is in negotiations to purchase the RD-180 rocket engine from its Russian manufacturer. These developments underscore the increased security cooperation between Russia and China, which could potentially threaten U.S. air superiority .