Context: Recently, Russia has promised to accelerate deliveries of some defence contracts with India, in a meeting of Indian Defence Minister and  Russian Deputy Prime Minister.
About S-400

• The S-400 Triumph  is an air defence missile system developed by Almaz Central Design Bureau of Russia. 
• The S-400 replaced the S-300P and S-200 air defence systems of the Russian Army.

Features: It is designed to destroy aircraft, cruise and ballistic missiles, including medium-range missiles, and can also be used against ground installations.

• The S-400 can engage targets at a Range of 400 km and At an altitude of up to 30 km.
• The S-400 Triumph air defence system integrates a multifunction radar, autonomous detection and targeting systems, anti-aircraft missile systems, launchers, and command and control centre. 
• It provides four different types of layered air defence.
• The system can simultaneously engage 36 targets.
• The missiles are launched by a 6×6 tractor truck Transporter-Erector-Launcher (TEL) vehicle. The TEL vehicle can carry up to four launch tubes holding a mix of missiles.
• It can also be integrated into the existing and future air defence units of the air force, army and navy.

Key Points

• India’s Defence Minister has requested Russia to speed up deliveries of the S-400 long range air defence system along with spares and support for military hardware.
• India’s Defence Minister is on a three-day trip to Russia to attend the 75th Victory Day which marks the end of World War II and the victory of the Allied Forces in 1945. Russia celebrates Victory Day on May 9 while Europe and USA celebrate it on May 8.
• S-400 deal: In 2018, India signed the deal worth $5.43-billion to buy five Russian S-400 Triumf missile shield systems.
• Booster to the Indian Air Force: Acquiring the missile system will help repulse the air attacks by India's adversaries, especially Pakistan and China.
• US opposition: The Trump administration had threatened to impose sanctions on the states that are acquiring weapons and military hardware from Russia under the Countering America's Adversaries Through Sanctions Act, CAATSA.

Major  Weapon  Systems  Purchased  from  Russia

• Su-30MKI  multi-role  fighter  aircraft
• Il-78 tanker aircraft to be used as platform for Airborne Warning and Control System(AWACS)
• Mi-17-IV military transport helicopters
• R-77 air-to-air missiles
• Kilo  class/type  877E  submarines,  frigates
• Ka-31  Helix  airborne  early warning helicopters,
• Aircraft carrier Admiral Gorshkov (INS Vikramaditya)
• MiG-29K, including MiG-29KUB version for use on aircraft carrier 
• Ka-27PL(Ka-28  version)  and  Ka-31  helicopters
• T-90  tanks

Major Joint Military Programme between India & Russia

• BrahMos cruise missile programme
• 5th generation fighter jet programme
• Sukhoi Su-30MKI fighter jet programme (230+ to be built by Hindustan Aeronautics)
• Ilyushin/HAL Tactical Transport Aircraft
• KA-226T twin-engine utility helicopters
• India entered into a joint venture with Russia to manufacture the legendary Kalashnikov assault rifles in India.