Unless you mean trade war or mean words, then nobody.
The US navy and airforce have such superior firepower that it would be a disaster for anyone trying to actually fight you.
To top it off, the European Union isn't exactly popular. It primarily gets away with shady shit because nobody really knows about it.
Another question, different despite the same words:
If the US STOPPED ALL FLIGHTS to and from China, how many countries would fight us?
Unless you mean trade war or mean words, then nobody. The US navy and airforce have such superior firepower that it would be a disaster for anyone trying to actually fight you. To top it off, the European Union isn't exactly popular. It primarily gets away with shady shit because nobody really knows about it.