On 22 June 1941, the Sisak Partisan Detachment was formed in Brezovica forest near Sisak; this was to be celebrated as the first armed resistance unit formed in occupied Europe during World War II. Croats, Serbs, Bosniaks, and citizens of all nationalities and backgrounds began joining the pan-Yugoslav Partisans led by Josip Broz Tito. The Partisan movement was soon able to control a large percentage of the NDH (and Yugoslavia) and before long the cities of occupied Bosnia and Dalmatia in particular were surrounded by these Partisan-controlled areas, with their garrisons living in a "de facto" state of siege and constantly trying to maintain control of the rail-links.