War with one EU member is war with the entire EU. The European Union's constitution plainly states that if any member is attacked, all others are required to do everything in their power to help defend that member.

As opposed to NATO's Article 5 that only requires all other members to do what they deem is necessary, which may be almost nothing at all.

Of course, divided as the EU currently is, we probably can't expect it to suddenly work together as one to defend from American aggression. Even pro-EU countries may end up doing nothing due to various factors such as pro-US president (Poland) or major American military equipment contracts (Finland).

At the very least France seems prepared to defend the European Union. As much as I criticized Macron in the past, I'm happy to see him standing up to the empire and firmly supporting the EU.