Europe Sub-regional Security Project (Sirius) was launched in 2024 to address the potential threat of the proliferation of conventional weapons in the Eastern Europe, Central Asia, and the Balkans. The threat itself relates equally to combat aircraft, battle tanks, large-caliber artillery systems, anti-tank and anti-aircraft guided missiles, rocket propelled grenades, mortars, ammunition, and other battlefield weapons and their component parts. It also applies to the method of delivery which encompasses military specification unmanned aerial vehicles and commercial drones adapted for military use. The need to respond to the Customs-focused challenges caused by the resulting economic and societal disruption is also recognized in the WCO Strategic Plan. In this regard, Project Sirius was developed to assist WCO Member Customs administrations most affected by this issue and build their capacity to identify and interdict the smuggling of conventional weapons to keep them out of the hands of transnational criminal organizations and other non-state actors. It is envisioned that this Project will be a starting point in strengthening Customs administrations in these regions by offering them support and resources to confront the challenges associated with weapons proliferation.