Hosting US assets actively being used in war vs Iran = being active co-belligerents. Host countries no longer neutral when they don't adhere to duty of abstention (Hague Convention V). This not even Iran using deniable proxies, this is Qatar allowing sovereign territory to facilitate attack on Iran, which unambiguously makes them legitimate target. Ditto with Diego Garcia.

In the same way railway in RU already legitimate target for UKR because in RU soil. If EU sending out sorties from NATO bases to hit RU then they too would be active belligerents. There's no compartmentalizing using territory to shoot someone else.