Solidarity clause
NATO and the Western European Union have mutual defence clauses in art. 5 of both treaties.
According to art. 5 NATO, an armed attack against one of the signatories shall be considered an attack against them all. Therefore, in line with art. 51 of the UN Charter, such attack would lead to a right of “self-defence” of all NATO member states.
Art. 5 WEU states that the signatories shall support the attacked state in its right of self-defence with all the military and other aid and assistance in their power”.
The Lisbon Treaty has a solidarity clause against terrorist attacks and disasters in Article 222 TFEU and a mutual assistance clause in Art. 42.7 TEU in case of an armed aggression by another state.
This clause is similar to Art. 5 of WEU.
Member states decide on their own how they will deliver the solidarity. There is always unanimity on defence issues.
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