Well covered on Leeham News last week:
https://leehamnews.com/2019/09/29/eu...rbus-wto-case/
https://leehamnews.com/2019/09/30/eu...n-outstanding/
TL;DR If the USA imposes any tariffs, the EU already has the WTO-given right to retaliate.
Does the US Government really want to open a second front in the trade war?