Armenia admits that it has its own S300 anti-aircraft missile systems. This has been rumored for over a year, but this was the first official admission. Since the 1990s, Russia has made no secret of that fact that it's 3,000 man force in Armenia (as part of a "friendship treaty") was equipped with S300 systems, as part of a joint air defense system for the CIS (Commonwealth of Independent States, a coalition of some of the countries that used to be part of the Soviet Union). With Armenia now controlling some S300s of its own, neighboring Azerbaijan is less likely to revive the dispute it has with Armenia over some territory. That war ended in 1994, but Azerbaijan was a poor loser and threatens to try again.
Meanwhile, last year Armenia signed a pact with Russia that, in effect, makes it a protectorate of Russia. The deal extends the lease on a Russian military base in Armenia from 2020 to 2044. The 3,000 man Russian force in Armenia may be increased and Russia, in effect, guarantees Armenia's security. Armenia needs all the help it can get, as it is a landlocked Christian nation surrounded by three hostile Moslem states (Turkey, Azerbaijan and Iran). To the north, there is Georgia which, while Christian, has its own problems with Russia.
There are over nine million Armenians worldwide (including the three million still in Armenia), making it clear that most Armenians want to be anywhere but Armenia. But not everyone could get out. Moreover, Armenians have been in Armenia for over 3,000 years, and there is a certain attachment to the place. The Russians like surrounding Georgia, and being closer to Turkey (an ancient enemy).
In return for this security, Armenia will have to follow Russia's lead in diplomacy, and any other area the Russians feel is important (like relations with Azerbaijan, a friend of Russia). Meanwhile, the Russians will provide new weapons and equipment for the 43,000 troops in the Armenian military, and help arm an even larger reserve force.
The only active enemy Armenia has at the moment is Azerbaijan. Both countries continue to disagree over possession of Nagorno-Karabakh, a 4,400 square kilometer district, full of Armenians, surrounded by Azerbaijani territory. Technically, there has been a truce between Armenia and Azerbaijan since 1994. But it has been a hot truce. Between 1991 and 1994 there was a war between the two countries over Nagorno-Karabakh, which Armenia won. Some 20,000 people died, and over a million (400,000 Armenians and 700,000 Azerbaijanis) fled their homes as Armenia occupied 31,000 square kilometers of Azerbaijani territory, to connect Nagorno-Karabakh to Armenia. Most of the refugees were from areas dominated by one group, who drove out the minority. Some 40,000 Azerbaijani civilians were driven from Nagorno-Karabakh. The situation was humiliating for Azerbaijan, who saw it as yet another example of more powerful and wealthier (via oil fields) Moslems being defeated by a smaller number of more capable non-Moslems.
The Armenians have survived, although surrounded by Moslems, for centuries. But the Armenian economy is a disaster, particularly since Turkey and Azerbaijan have closed their borders with Armenia. Since the early 1990s, the best educated Armenians have been emigrating. They join a six million strong community of expatriate Armenians. This group can raise millions of dollars on short notice, and have provided the emergency funds when needed for the fighting against Azerbaijan. Some twelve percent of the 150,000 Armenians in Nagorno-Karabakh are armed and organized to defend the mountainous area, and are backed up by even more troops in Armenia. Since the late 1990s, the economy began recovering as well, and is now growing at about 7 percent a year, but is still poorer than its major neighbors (Turkey, Iran and Russia)
Azerbaijan is making a serious effort to create an effective military. Azeri defeats at the hands of better trained, led and organized Armenian troops were caused, in part, by Azerbaijani corruption and double dealing among themselves. Moreover, the Armenians have a military tradition going back centuries. The Azeris are constantly working to redress the military balance, thus the Armenian need for a Russian alliance.
The S-300 system is roughly equivalent to the U.S. Patriot system and 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 8.4 meters (26 feet) long and about 50 cm (20 inches) in diameter. The missiles have a range of some 200 kilometers and can hit targets as high as 32 kilometers (100,000 feet). The missile has a 145 kg (320 pound) warhead.