India's decision to buy Russia's S-400 missile system has left the US uncomfortable. The US had made it fairly clear that defence ties between India and the US could be impacted if New Delhi goes ahead with the purchase of S-400 Triumf air defence missile systems. Despite this, India has decided to go ahead with the deal.

India wants to procure the long-range missile systems to tighten its air defence mechanism, particularly along the nearly 4,000-km-long Sino-India border. In fact, Russia and India are likely to announce the deal before an annual summit between Prime Minister Narendra Modi and Russian President Vladimir Putin in October.

There can be two possible reasons why the US is unhappy with the deal. One is that since India has emerged as the largest importer of military equipment in recent years, the US keen to become New Delhi's main supplier. Another is that S-400's comparable missile system, US made MIM-104 Patriot, is losing market to the Russian weapon system across the globe.

Both are surface-to-air systems designed to shoot down aircraft and ballistic missiles. But the S-400 has a longer range than the Patriot, 250 kilometres (155 miles) vs. 160 kilometres. An upgrade coming later this year is expected to stretch the S-400's range to 400 kilometres.

It also has a more powerful radar, can destroy targets moving twice as fast, and is quicker to set up. While both systems are mounted on trucks, the S-400 can be ready for action in five minutes, compared with an hour for a Patriot battery. The S-400 is also slightly cheaper than the Patriot, on a per-battery basis.

All you need to know about S-400:

The S-400 Triumf is an anti-aircraft weapon system developed in the 1990s by Russia's Almaz Central Design Bureau as an upgrade of the S-300 family. It has been in service with the Russian Armed Forces since 2007. The S-400 uses four missiles to fill its performance envelope: the very-long-range 40N6 (400 km), the long-range 48N6 (250 km), the medium-range 9M96E2 (120 km) and the short-range 9M96E (40 km). The S-400 was described by The Economist in 2017 as "one of the best air-defence systems currently made."

Development of the S-400 system began in the late 1980s, and the system was announced by the Russian Air Force in January 1993. On 12 February 1999 the first, reportedly-successful tests were performed at Kapustin Yar in Astrakhan, and the S-400 was scheduled for deployment by the Russian army in 2001.

In 2003, it became apparent that the system was not ready for deployment. In August, two high-ranking military officials expressed concern that the S-400 was being tested with "obsolete" interceptors from the S-300P system and concluded that it was not ready for deployment. Completion of the project was announced in February 2004, and in April a ballistic missile was successfully intercepted in a test of the upgraded 48N6DM missile. In 2007, the system was approved for service.