Ever since India signed an Inter-Governmental Agreement (IGA) with Russia for buying five regiments of the S-400 Air-Defence missile system. America is warning India over CAATSA sanctions but India has been holding ground in response to CAATSA sanctions.
India formally signed a deal of 40,000 crore Rupees on 5 October 2018 with Russia ignoring the threat of US sanctions. The United States is threatening India because India chose the Russian S-400 over the American Patriot PAC-3. So the threat is not over some world peace or something, it is totally based on business. Since America is a capitalist country, they want to reduce their competition as much as they can in the global market.
Capabilities of S-400 Air defence missile system :-

This air defence system is capable of blocking enemy aircraft and also provides protection against nuclear missiles. Having the maximum range of 400 Km and the minimum range of 40 Km, it can simultaneously engage 36 targets and can control up to 72 launchers with a maximum number of 384 missiles. It can attack one target with 2 missiles.
It is also capable of destroying fighter aircraft like F-15, F-16 and also able to destroy ballistic missiles.
The S-400 has been used in the 2022 Russian invasion of Ukraine. The S-400 shot down Ukrainian Su-27 over Kyiv. On 14 April 2022, the Russian Defence Ministry claimed that an Ukrainian Mi-8 was shot down by the S-400.
Development :-
It began in the late 1980s and was announced in January 1993. the completion of project was announced in February 2004. Since 2007, it is servng Russia.
Since then it has been serving Russia and protecting its airspace and now it will serve the Indian Air Force.
Current operators of the S-400 air-defence missile system are :-
Currently many counties are using this missile system because of its vast range and capabilities.
The countries which currently operate this air defence system are :-
- Russia
- China
- India
- Turkey
- Belarus