In 2015, the Netherlands and Sweden provided a joint contribution to the EU's counter-piracy military mission EUNAVFOR Operation Atalanta. During their three-month deployment to the area of operation, Swedish troops and enablers - including two Combat Boat 90 assault craft and two AW109 helicopters - were stationed on board the Dutch warship HNLMS Johan de Witt, which also hosted the Force Headquarters (FHQ) led by a Swedish Admiral. This kind of cooperation, in particular having a tactical headquarters led by one nation and the flagship led by another, was quite unique. In general, the integration was considered to have been successful - to some extent surprisingly so. This report describes and analyses the planning and execution of the fusion of Dutch and Swedish forces, identifying key lessons that may be of value in similar future collaborations. National regulations and procedures, command and control structures, preparatory training and exercises, the chosen level of integration and personal mindsets are among the issues discussed.