Article 42.7 of the EU Charta would also apply and is a bit more direct:

If a Member State is the victim of armed aggression on its territory, the other Member States shall have towards it an obligation of aid and assistance by all the means in their power

https://www.europarl.europa.eu/meetdocs/2009_2014/documents/sede/dv/sede200612mutualdefsolidarityclauses_/sede200612mutualdefsolidarityclauses_en.pdf