If the US doesn't sanction India, then that is a pragmatic...

If the US doesn't sanction India, then that is a pragmatic decision, based on a bag of perceived positives which They consider outweigh the disadvantages for the US.
The S-400 system is a world beater and a game changer. The US will not want to see its proliferation In the hands of opposing and non-aligned countries.
Trump fancies himself as a shrewd operator. He uses deception on a daily basis. Witness all his BS with North Korea.
The downside of this is that he destroys trust. Increasingly, Nations are going to weigh up very carefully where their best security interests lie, and many over time are going to beat a path to Moscow to get the S-400.
Given its superiority, I am personally surprised that Russia has decided to make it available, particularly to Turkey. Clearly, Turkey must have moved very close to Moscow.
Very interesting fast moving times.
23.