They can. They don’t though. I advocate that they do. I’d love to see the EU with its own defensive force

Article 42

And

The command was designed in light of growing hostilities between European countries and Russia since the annexation of Crimea in 2014 and in response to logistic and bureaucratic hurdles limiting military logistics in case of a crisis.

https://en.m.wikipedia.org/wiki/Joint_Support_and_Enabling_Command