Summary: Article 42.7 of the Lisbon Treaty commits member states to aid and assist others that are the victim of armed aggression on its territory. Article 222 develops this concept of solidarity, expanding the triggers for assistance to include terrorism and natural disaster while framing the defence concept. France invoked it in 2015.
At stake: This creates an ‘Article 5’ (NATO) concept for EU mutual defence. The Solidarity Clause has already been shown to be open to abuse by the UK being dragged into its application for the Eurozone bail out, self-evidently neither a natural disaster nor a hostile act.
What they say: “once a Member State has decided to invoke the clause, it should not be a matter for debate for the others to offer assistance; […] solidarity also entails an obligation to invest in adequate national and European capabilities” – EP Resolution 2012/2223
Exposure: Should end with Brexit, but there is a risk of implied commitment if the UK reassumes a seat on EU bodies.
Action: UK to assert it will only consider itself bound by NATO obligations, and to avoid any association with EU activity framed by Lisbon treaty commitments (ie it will avoid participation in EU institutional bodies).
Deep dive: http://www.theredcell.co.uk/uploads/9/6/4/0/96409902/eu_defence_nouvelle_vague_rotherham_thompson.pdf