In my opinion it's because of:

- nuclear deterrence - like other commenters said, no one really wants a nuclear arms race, but I think it's bound to happen. Poland, the Baltics, Sweden/Norway will probably need to develop their own nuclear deterrence, and obviously Ukraine.

- the US for the most part has been a good friend and ally, and has a lot of weight, and above all it helped to create the EU and wanted the EU to succeed.

- Europe has had a lot of conflicts, and a lot of parties funded by foreign states are seeking to bring EU down, including the US administration. It would be nice for Europeans if it wasn't like that.

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