"On the one hand, there have been more and more signs that China will become the first foreign customer of Russia's most advanced anti-missile system, the S-400. China's People's Liberation Army already operates the Russian-made S-300 anti-missile system, and Beijing and Moscow have reportedly been negotiating over the S-400 since 2010. These talks have reportedly been slowed by a number of issues, including Russia's concern that China would reverse-engineer the advanced anti-air and anti-missile system."
Scratches to Sketches