Indian plans to acquire the Russian missile defence system S-400 are likely to mar US-Indo ties. The US Congress had passed the Countering America’s Adversaries Through Sanctions Act (CAATSA) in 2017 stipulating the imposition of sanctions on any country that purchases military equipment from Russia or conducts transactions with Iran or North Korea. The US had already imposed sanctions on Turkey for its acquisition of the S-400 Missile System from Russia and as a punitive measure, discontinued Turkey’s participation in the development of the F-35 advanced fighter jet programme. It must be remembered that Turkey is a valued ally and an important regional security partner for the United States, yet it faced economic sanctions for accepting the Kremlin’s missile system. Despite the US-Indo ties of warmth and bonhomie, the US has warned India that it will impose sanctions if it goes ahead with the purchase of S-400. In January 2021, the US has expressed its displeasure at the deal and had issued a warning and had expressed its concerns again during the US Defense Secretary, Lloyd Austin’s, visit to New Delhi in March 2021. The S-400 is an advanced missile defence system, which comprises an array of radar systems and missile pads. The Long-range surveillance radar tracks objects and relays information to the command vehicle, which assesses potential targets. After identification of the target as a hostile intruder, the command vehicle orders missile launch. The data for the launch is sent to the best placed launch vehicle, which releases surface-to-air missiles. The engagement radar helps guide missiles towards the target. At the moment, it is a contest of wills where the US is pressuring India to abandon the deal while the latter has refused to back down from it The Missile Defence System has a 400 km range and can engage and shoot 80 targets simultaneously. It can detect and destroy incoming bombers, airplanes and ballistic missiles. Induction of the S-400 systems would tremendously boost India’s air defence, as well as missile defence capability. India already has its indigenous two-tiered missile defence system based on Prithvi Air Defense for high altitude interception and Advanced Air Defense system for lower altitude interception. The Indian Missile Air Defence System is now operational and can provide protection to two major Indian cities – New Delhi and Mumbai. The acquisition of the S-400, if it goes ahead, will further strengthen India’s missile defence capabilities. This will have a direct effect on the nuclear deterrence equation between India and Pakistan. The resulting imbalance is theoretically going to render India invulnerable to incoming missile attacks, which may embolden the Indian defence planners to indulge in adventurism against Pakistan like it did on February 26, 2019, which had backfired. The US is not being cognizant of an arms imbalance in the region but is desirous of taking retributive action against Russia, denying it the opportunity to sell defence equipment. Apparently, this was the main stimulus for the US Defence Secretary Lloyd Austin while discussing the proposed Indian acquisition of the Russian S-400 with his Indian counterpart, Rajnath Singh, on March 20. When asked a direct question if the US would sanction India if it proceeded with the acquisition, his response was that the question of sanctions against India is not on the table as India has not taken delivery of the system. He did not elaborate on what will happen when India does receive some S-400 systems later this year. Readers may recall that earlier this year, in January 2021, a US embassy spokesperson in Delhi issued a similar warning over S-400 plans by India. He had warned all US allies and partners to forgo transactions with Russia that risk triggering sanctions under the CAATSA. He had elaborated that the CAATSA does not have any blanket or country-specific waiver provision. India, which has been relying on Russian defence equipment for decades, will be badly hit if sanctions are imposed by the US. In its defence, India has argued that the process for acquisition of the S-400 system began in 2016, before CAATSA was introduced in 2017 by the Trump administration. The Indian deal for five S-400 systems worth US$5.5 billion was inked on October 5, 2018, between Russian President, Vladimir Putin, and Indian Prime Minister, Narendra Modi. All five S-400 regiments were scheduled to be delivered between October 2020 to April 2023. The S-400 is one of the most advanced air defence systems in the world and India has already made a payment of $800 million in 2019 toward the S-400 deal and the first set of missile batteries are expected to be delivered by end of 2021. Since its improvement of ties with Washington DC, New Delhi has edged closer to the US and Israel for acquiring weapon systems. The US has emerged as one of India’s biggest arms sellers, in the near past. India has plans to buy armed drones from the US as well as a large order for over 150 combat jets for the air force and the navy to help build-up its conventional capabilities. However, the Indian S-400 purchase has cast a shadow over the budding Indo-US Strategic Partnership. The US officials – both in the outgoing Trump administration and the new Biden Administration have warned India that going ahead with the deal will trigger CAATSA sanctions. At the moment, it is a contest of wills where the US is pressuring India to abandon the deal while the latter has refused to back down from it. The US must be in a quandary over sanctioning India for the S-400 purchase under CAATSA, for which a precedent has been set against Turkey. As per the CAATSA act, the US must sanction India on the purchase of the Russian S-400. The caveat that the US faces is that India is the cornerstone of America’s Indo-Pacific strategy and a founding member of the Quad established to counterbalance Chinese influence in the region. It would also jeopardise future arms transactions between the US and India such as high-end fighter aircraft and armed drones. The writer is a retired Group Captain of PAF. He is a columnist, analyst and TV talk show host, who has authored six books on current affairs, including three on China