Constitutionally, Belgium represents the most extreme case of regional entities wielding power over EU external trade policymaking. Formally, the Flemish, Walloon and Brussels regions can wield veto power over Belgian positions. Yet, only once has a Belgian region actually made use of this capacity, when Wallonia temporarily blocked the conclusion of the EU trade agreement with Canada in 2016 (CETA). We show that political actors - legislative and executive - could only activate this constitutional possibility in conjunction with other necessary conditions: a high degree of societal mobilization and, above all, inter-party competition across different levels of government. As the Walloon Parti Socialiste seized the moment, it reinforced the paradox of weakness and strengthened the EU's trade bargaining power towards Canada. We finish by discussing the spill-over effects of the 2016 CETA episode into the shaping of future EU trade policies, as well as into future intra-Belgian EU policymaking.