No, that was a result of the Clean Air Act Amendments of 1990 for the reduction of acid rain. Though it was targeted towards industrial emissions of SO2 & NOx, but stricter regulation for vehicles were an additional effect.
>specifically disadvantaging European diesel cars
That's a weird argument given that diesel passenger vehicles in the US are held to the same standard as gasoline ones, but to a separate standard from their petrol counterparts in the EU. I mean, one could argue the opposite, that an EU emissions policy favorable to diesels amounted to an equivalent 13-16% import tariff. [1] Several domestic rather than just foreign diesel engine manufacturers were also penalized for using defeat devices in 1998.[2]
> (which are much cleaner overall)
That's quite arguable, trading lower CO2 & CO for increased NOx & PM.
[1]: http://www.eugeniomiravete.com/papers/MMT-Diesel.pdf
[2]: http://articles.chicagotribune.com/1998-10-23/news/981023011...