Just a thought, could it be that the US was looking for an excuse to not hand over the f35 to Turkey, and the s400 came in handy, because of Israel? I mean the tug of war between Israel and Turkey has been going on for quite some time now (I think since that Ghaza boat incident), given that Erdogan's master plan is to rebuild the Ottoman Empire, and Israel was part of the Ottoman Empire. This is a recent bit of the Turco-Israeli feud:
https://www.pappaspost.com/erdogan-t...anyahu-junior/
Now, I see that as a threat. But given that Turkey is quite prone to making empty threats diachronically
, the US is content to enforce non-violent measures against Turkey (wrecking its economy by means of tweets, cutting it out of projects it worked on, such as the f35, and even trolling Turkey enough to propose selling f35s to Greece
https://www.reuters.com/article/us-u...-idUSKCN1RG2OF (I hope not, I consider the f35 to be one expensive bucket, it's buggier than, well, let's not name titles here, and it still has a long way to go in order to be a good airplane).
Of course, the US can't just say that we are disciplining a NATO member in order to defend a non-NATO member.