Manufactured by Russia, the S-400 surface-to-air missile system is considered as one of the most advanced air defense systems in the world.

It poses a direct challenge to America's Patriot and THAAD systems. The S-400 is a massive upgrade on the S-300, which was earlier used in the war in Syria. Its radar has a range of at least 600 kilometers-per-second for surveillance and 400 kilometers-per-second for missiles, according to military experts.

The missile system is also capable of tracking large numbers of potential targets. Another advantage is its high mobility, which means it can be set up easily, fired and moved within a few minutes. The S-400 also provides more options to its buyers due to its one-size-fits-all system. It can be configured with long, medium and short-range weapon systems.

The S-400 is also Russia's most marketable product. So despite opposition from the U.S., many countries, including American allies like Saudi Arabia and Qatar, have also shown interest in buying the military hardware. In 2018, China and India signed agreements with Russia to purchase the weapon systems. Turkey's 2 billion U.S. dollar deal with Russia is facing stiff opposition from the U.S. and NATO. As a member of NATO, Turkey boasts the second-largest army within the military organization. Washington says S-400s are not compatible with NATO's defense network. It also fears that if the Turkish military operated both the F-35 stealth fighter jets and S-400, it would allow Russia to access sensitive information about the advanced jets.

The U.S. has threatened Ankara with economic sanctions and expulsion from the F-35 program. Under the program, Turkey signed up to buy 100 F-35s, with Turkish companies producing over nine hundred parts of the planes. Many believe the move could deal a significant blow to Turkey's defense sector.

So far, Ankara has refused to buckle under U.S. pressure. It argues that given an unstable geopolitical scenario in the neighborhood, the S-400 is a critical addition to its defense capabilities. Turkey also says the deal would help the country's defense industry become more self-sufficient.