It is afraid that the EU seems to become apart from the US in its diplomatic policy.
Angela Merkel, the chancellor of Germany, declared that Europe would not be able to rely on the US for protecting the safety, recently. Emmanuel Macron, the President of France, agreed with her opinion.
StraisTimes: Europe can't count on US protection any more: Merkel
In the era of cold war, Western European nations collaborated with the US as members of NATO. But it was because there is an arch enemy: Soviet Union. In recent years, the US behaves, especially led by Trump administration, independently from NATO. Trump has declared that he would not guarantee the international safety whereas the US had taken the role of the world police.
Nonetheless, some dominant nations in Europe such as the UK and Germany acted supportively to the US. But recently diplomatic policy of Trump administration is apparently unfriendly to European nations. Therefore, Germany and France have to be prepared to the conflict with estimated enemies such as Russia and China without the support of the US.
To begin with, the rise of ISIS is partially attributable to the wrong policy of the US in the past. As a result, many European people were victimized by the terror attacks. Facing the withdrawal of the UK, the EU will encounter a difficult situation.