The evolution of the European Union, far from being linear, is undoubtedly a story of laudable advances and resounding setbacks. This uneven evolution is particularly visible in the area of security and defence. Thus, if on the one hand the Treaty of Amsterdam was followed by years of great advances such as those that materialised in Saint Malo, Helsinki, Cologne, Santa Maria de Feira and Lisbon, then the triple shock of the Iraq War, the financial crisis and the COVID 19 pandemic led to a period of deadlock and a substantial darkening of the outlook. However, this situation would not be in doubt forever because a new war, in Ukraine, perceived as a fundamental strategic change that challenged the interests and values of the Union, was to awaken Europeans and generate a new momentum in terms of security and defence, allowing for a more determined search for greater strategic autonomy with all that this entails. This momentum is further boosted by the fact that the EU's easternmost member states, traditionally more reticent in this area, being the most affected by the renewed Russian threat, may start to be much less so. In these conditions, the enlargement debate is even reopened, opening the door to states that, if they are able to meet the preceptive requirements for accession, could in some cases constitute an undoubted asset for the security and defence of the EU as a whole.