S-300 missiles — which are useless against incoming ICBMS.
Only the Russians have effective defenses against mass ICBM attacks and have been uprgading those systems since the mid-60s when they went operational. Nixon knew the Anti-ICBM Treaty was a farse when he signed it - good for political points at home and useful to stop real manned space flight endevours using nuclear power.
S300-S400 are the export models (and descended from) another secret not exported system used for defense against mass ICBM attacks.
The S-300 is a series of Russian long range surface-to-air missile systems produced by NPO Almaz, all based on the initial S-300P version. The S-300 system was developed to defend against aircraft and cruise missiles for the Soviet Air Defence Forces. Subsequent variations were developed to intercept ballistic missiles.
http://www.fas.org/nuke/guide/russia/airdef/s-300v.htm