The initiative for trilateral co-operation between Slovenia, Hungary and Italy was launched by Slovenia in 1996, becoming, with the admission of Croatia in September 2000, the Quadrilaterale. It is a form of concrete co-operation between countries situated in the same geopolitical area and participating in joint projects. In terms of content, co-operation is underway in the fields of internal affairs, defence, culture, environment, regional development, labour and employment. In 2010 the participating countries agreed to close down this form of collaboration as it has met the objectives.
The 2003 Quadrilaterale was chaired by Slovenia. During its chairmanship, Slovenia focused particularly on cross-border co-operation, the fight against organised crime and illegal migration, the development of an information society, the construction of Pan European Corridor V and culture. In cultural co-operation it focused on the project of publishing and translating literary works in the languages of member states of the Quadrilaterale. In January 2004 a successful meeting between the four prime ministers of Slovenia, Italy, Hungary and Croatia took place in Slovenia.