Giving details about the losses sustained by Pakistan during Operation Sindoor for the first time, Air Chief Marshal A P Singh on Saturday (August 9) said the Indian Air Force (IAF) took down “at least” five Pakistani fighter jets and “one large aircraft”, and that the S-400 missile system, bought from Russia, had been a “game-changer”.The kill range of the S-400 system kept Pakistani aircraft away, the IAF chief said.Singh’s praise of the Russia-supplied weapons comes at a time India is facing remarkable heat from the US for its ties with Moscow. Interestingly, when India bought these weapon systems from Russia, the cloud of possible US retaliation was hanging over the deal, but New Delhi went ahead with it.What makes the S-400 missile system so valuable? What risk of US action was India facing when it made this weapons deal with Russia, and what happened after? We explain.What is the S400 missile system?The S-400 Triumf, which NATO calls it the SA-21 Growler, is a mobile, surface-to-air missile system (SAM) designed by Russia. Made operational in 2007, it is seen as the most dangerous operationally deployed modern long-range SAM in the world, better than the US-developed Terminal High Altitude Area Defense system (THAAD).Put simply, its job is to detect any aerial threat in the area it has been tasked with protecting, calculate the threat’s path, and then dispatch a suitable missile to counter the threat.The S-400 has a multifunction radar, autonomous detection and targeting systems, anti-aircraft missile systems, launchers, and a command and control centre. It can be deployed within five minutes — US-made systems often take longer to deploy — and is capable of firing three types of missiles to create a layered defence.Story continues below this adExplained | Operation Sindoor’s 300-km kill: How the Indian Air Force may have set a rare recordIt can engage all types of aerial targets including aircraft, unmanned aerial vehicles, and ballistic and cruise missiles within a range of 400 km, at an altitude up to 30 km. This makes it more versatile than the American THAAD, which is most effective against ballistic missiles.The S-400 can also simultaneously track 100 airborne targets, including super fighters such as the US-built F-35, and engage six of them at the same time.When, how did India buy S-400s?India bought the air defence system in 2018, and delivery began by 2021.In 2017, US President Donald Trump, in his first term, had signed into law the Countering America’s Adversaries Through Sanctions Act (CAATSA). Targetting Russia, Iran, and North Korea, the Act had provisions to sanction those who did significant business with these three countries in key sectors, inclduing defence and intelligence.Story continues below this adIndia’s deal for the weapons, thus, potentially fell foul of the Act. Even today, Trump is trying to punish India for doing business with Russia through tariffs.However, New Delhi had even then signalled that India would continue to use its strategic autonomy to make purchases for its security goals. Eventually, India was not sanctioned under CAATSA, as Washington was seeking to build on its ties with India to counter China, and also because it wanted to sell more weapons to India. New Delhi has had close defence cooperation with the Soviet Union since the Cold War days, but has continued to buy weapons that suit its needs from across the world, including US-made arms.