Stories by Yarno Ritzen of Al Jazeera

The Russian weapons system dividing the U.S. and key allies

A soldier looks at a radar screen of an S-400 missile system at the Vostok 2018 military exercise, which involves troops from China and Mongolia. Photo: TASS via Getty Images

Military hardware is Russia's second-biggest source of income after oil, and the S-400 surface-to-air missile is one of the country's most advanced and marketable weapons systems in recent years.

Why it matters: Because of its capabilities, several countries including China, Saudi Arabia, Turkey, India and Qatar have said they are willing to buy the S-400. Almost every government that announced it was planning to buy the system was threatened with some kind of diplomatic retaliation from the U.S., NATO or adversaries. The reason for this blowback, according to several experts Al Jazeera interviewed, is not only because the S-400 is technologically advanced, it also poses a potential risk for long-standing alliances.