There's actually a strongly worded clause in the Treaty of Lisbon, §42.7:
"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, in accordance with Article 51 of the United Nations Charter." https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=celex%3A12016M042