The Bucharest 9 (B9), a group composed of NATO’s easternmost members, has quickly earned clout as the voice of states whose security is the most undermined by the increasingly provocative Russian rhetoric and force posture and of the region that has become the focus of the Alliance’s response to this threat. Indeed, the potential of the B9 to shape the NATO agenda is significant, not least because most of its countries present a rigid commitment to common defence by taking on an increased burden in NATO by increasing defence expenditures and investing in new capabilities. Yet, the B9 also faces limits to its effectiveness because of its participating states’ differing threat perceptions, uneven commitments to beefing-up national defence capabilities, and the potential volatility of their respective military modernisation and transformation plans. More cooperation within the B9 framework, both political and military, could alleviate these problems and help make it the real voice of the Eastern Flank.