IADS is an acronym for an integrated air defense system. Such a system commands, controls, and coordinates air defense assets: radar, visual observation, fire controllers, surface to air missiles, anti-aircraft weapons, and in many cases friendly combat aircraft. The Russians have generally invested in such systems that rely heavily on surface-to-air-missiles–SAMS.
Russian designed and built IADS systems have not worked very well since the initial stage of the the 1973 Yom Kippur War. Early in that war, Israeli fighter bombers conducted their attacks against Egyptian and Syrian forces protected by Russians IADS systems, as they had struck in their past wars. The IDF struck the Arabs from the air as they had since 1948, with abandon. The Israelis learned the hard way that one could not simply bore in on a target protected by an IADS without suffering heavy losses. The IDF quickly learned that lesson, changed their methods of attack, and by the end of the war were operating successfully without taking heavy losses.
Russian IADS systems also failed in Libya in 1986, and during the 1991 First Gulf War. The Coalition air forces operated against Iraqi Russian-built system with near impunity. In 1991 NATO did the same against Serbia and its IADS.
In recent years, on several occasions, Israeli aircraft, and in April of 2017 and 2018 American and Allied air assets, have likewise penetrated Syrian air space with few losses. (You are free to believe the statements of the Russians and Syrians that these IADS downed three-fourths of the incoming cruise missiles. I do not.)
This should be no surprise to anyone. IADS systems haven’t worked in a half-century! They do force an attacker to plan carefully and assign additional forces to insure mission success. But Russian IADS systems have never successfully defeated an enemy air attack. At best, in Vietnam and the Sinai in 1973, IADSs have inflicted more significant casualties on the attacker.
Nevertheless, the Russians keep selling these expensive systems to states around the globe. I understand why the Russians hawk their IADS systems. Moscow needs the cash. I do not understand why countries keep buying systems THAT DO NOT WORK! I suppose the answer is that, as expensive as they are, they are cheaper than the cost of building an American style air system, which is quite a bit more expensive and beyond the capability of many smaller nations. These states can afford the shiny Russians missiles, launchers, and support systems. They feel as if they are doing something to defend themselves. But the grim reality is: THEY DO NOT WORK!