Complex sentence: The bulk of the force had initially been organised in an improvised manner, but was formally reorganised in late September 1991 as the 204th Vukovar Brigade, also known as the 124th Brigade.
Split into simpler sentences: The bulk of the force had initially been organised in an improvised manner. In late September 1991, it was formally reorganised as the 204th Vukovar Brigade, also known as the 124th Brigade.