Many have discussed the crucial role that environmental nongovernmental organizations (ENGOs) have played in the implementation of nature protection policies across European member states. However, there are important differences in the opportunity structures among new and old member states that influence how ENGOs can act and undertake activities. This article seeks to clarify the role of ENGO capacity building within the context of multilevel environmental governance and focuses on a case in which Slovene ENGOs mobilized against the siting of 80 windmills in a natural area suggested for protection under the EU Birds and Habitats Directive. The dispute involved ENGOs seeking to pursue nature protection objectives against state authorities who prioritized green energy infrastructural development. The article analyses the mobilization strategies pursued and the combination of material, cognitive, social and symbolic resources used. The results suggest that these resources had to be mobilized and organized along both horizontal (domestic) and vertical (international) axes, and that this combination appears key in advancing an environmental protection agenda. Copyright © 2016 John Wiley & Sons, Ltd and ERP Environment