Washington has warned New Delhi that if India purchases missile system from Russia then this will jeopardize the defense cooperation and technology sharing between the US and India. This will also affect the interoperability between the two armed forces.
There seems to be too much stress between the US administration and the Congress for the S-400 missile. The chairperson of the armed forces asked India to make the decision carefully, considering all the factors before they purchase the S-400 advanced missile from Russia. Whosoever purchases this missile from Russia, the interoperability among the US forces and the respective country will be affected. The acquisition will further limit the comfortability to bring additional technology to the country who acquired this missile.
The decision to purchase S-400 from Russia may also affect the sales of US-built Predator drones.