In October 2022, the EU established a military assistance mission in support to Ukrainian armed forces (EUMAM Ukraine). The decision to launch the mission runs contrary to what one would intuitively expect from the EU, considering member states’ traditional divisions on Russia, alternative training initiatives, Ukraine’s lacklustre position and the mission’s unconventional design. Despite the growing literature on the EU’s response to Russian aggression, this puzzle has so far been overlooked. While a reformulation of EU member states’ security preferences driven by the heightened threat of an aggressive Russia is certainly necessary to explain the creation of EUMAM Ukraine, we argue that the launch and design of the mission can only be fully understood through a neofunctionalist approach, taking into account the effect of functional pressure and the activism of EU actors. The article shows how the EU High Representative for Foreign and Security Policy and the European External Action Service seized an initiative from member states and set the foundation for the mission of unprecedented scope and complexity. This finding illustrates the importance of Brussels-based actors in driving EU security and defence policy forward in times of existential security crisis in Europe.