Over the past decades, Europe has enjoyed an extraordinary period of peace, but this has not been achieved without any cost. Indeed, the membership of European states within the European Union and the North Atlantic Treaty Organization has played a key role, primarily through their commitment to mutual defence, contained in Art. 42 (7) of the Treaty of the European Union and Art. 5 NATO Charter. By establishing such clauses, the signatory parties commit to support each other in case of attack against any of them. Despite their importance in ensuring the security of all allies, scenarios such as the 9/11 terrorist attack on the US or the 2016 terrorist attacks in Paris showed their limitations when the affected states invoked these clauses.