Purchasing Russian missiles have risks of inviting US sanctions.

Since [India is officially a US ally][1], Why is India purchasing S-400 rather than the US-made missiles (Patriot or David's Sling or THAAD)?

If tech-transfer was the issue, S-400 doesn't come with a tech-transfer. Patriot and THAAD may also deny tech-transfer. In that case, Israel's David's Sling might have been a better option.

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**P.S.** My personal opinion is, India was probably trying to (1) avoid lengthy negotiation, (2) engage Russia with defense deals so that they don't lean or sell arms to Pakistan. 

  [1]: https://politics.stackexchange.com/q/54798/32479