That would not scare them, I do not think. They are very strong ideologues. Better to make them question what they claim their “values” to be, and speak the truth on issues that they do not let their people hear.
If Trump said that there would be quite a few people in Brussels in need of a change of clothes.
If Trump shut down trade and started pulling out our troops the Euro would drop like a stone. It would be worth 10 cents against the dollar in just a few days. Italy, Spain, Greece, France, Germany and more would declare they were no longer part of the EU.
It would be the near instantaneous end of the EU.