Pursuant to Common Article 1 to the four Geneva Conventions of 1949, the State parties have the obligation to respect and to ensure respect for International Humanitarian Law (IHL). In this context, the objective of this work is to analyze whether the European Union (EU) and two of its Member States – France and Spain – have enforced their obligation to ensure respect for IHL. It seeks to scrutinize how two juris corpuses, namely IHL and EU law, which used to follow separate paths, appeared to converge and be interlinked. This thesis takes the view that the enforcement of IHL must be analyzed from a multilevel perspective. While IHL rely on national law to be truly effective, the process of European integration makes it imperative to add the supranational level: the EU. This configuration generates a virtuous circle of compliance whereby the legal authority of Common Article 1 is reinforced, thus leading to the improved enforcement of IHL. In turn, the EU projects its values on the international scene and is strengthened as a leader in human rights. Likewise, the EU constitutes an additional level of guarantee and action for its Member States in the enforcement of their obligation arising under IHL. It is argued that the EU has established itself as an essential actor of IHL on the international scene. The EU – a self-proclaimed leader in human rights matters – and its Member States are not only bound by Common Article 1, but they also have accepted their mandate to effectively enforce it on the international scene.