On 27 July, an uprising broke out in the Krajina, triggered by the ambush and killing of a Croatian Home Guard officer. This had resulted in the rounding up and maltreatment of ethnic Serb villagers in the Bosansko Grahovo district. The local KPJ structures were not ready to launch an uprising, and had no instructions to do so. They were drawn into a mass Serb uprising not of their creation, and quickly had to adapt to their circumstances. The following day, an uprising broke out in the Romanija region, and this was followed by an outbreak of resistance in the Birač region on 5 August. The forces involved in this fighting were spontaneously formed companies based on the traditional social structure of villages, which were grouped into battalions when numbers dictated. Where the opportunity arose, KPJ members and military staff at the district and village level would attempt to gain some control over these units, but they were far from communist-led as a whole, at least initially. KPJ-led rebels across Bosnia-Herzegovina formed military structures as needed, without any central direction, and brigades and even a division were formed. In the Drvar district for example, units were known as Guerilla rather than Partisan detachments, and in some cases several companies formed a detachment, where in others, several detachments formed a company.