This is the first time the PLA has sent the J-20 in joint drills with another country, and this, along with other exercise arrangements like sharing PLA main battle equipment with the Russian troops, displayed a high-level of cooperation and mutual trust between the Chinese and Russian militaries at a time when the two countries are facing challenges from the changing security situation in Central Asia following the US' troop withdrawal from Afghanistan and direct threats from the US and its allies, Chinese experts said.