Russia and Belarus have agreed to create a joint air defense system to protect Belarus (and a chunk of Russia's western border as well). According to the announcement, this will involve anti-aircraft missile units, as well as interceptor aircraft.
This deal has other benefits for Russia. Belarus still operates several of the Soviet Union (pre-1991) era air-defense schools, and many Soviet era air defense installations. Three years ago, Belarus received 24 Russian S-300 missile systems. Roughly equivalent to the U.S. Patriot, S-300 was known as the SA-10 to NATO, when the system first appeared in the early 1980s. S-300 missiles weigh 1.8 tons each and are 26 feet long and about 20 inches in diameter. The missiles have a range of some 200 kilometers and can hit targets as high as 100,000 feet. The missile has a 320 pound warhead.
Belarus and Russia are integrating their air defenses, mainly to give Russia more "depth" in case of an attack from the west. Russia talks a lot about possible attacks from the west. No one in the West is quite sure what the Russians are talking about. Meanwhile, of all the former communist nations in Europe, Belarus is the only one still run by its Soviet era officials. Basically it's a police state, and very tight with Russia. The integrated air defense deal has been under negotiation for years, with the Belorussians demanding more goodies than the Russians were willing to let go of. But now the deal has been done.