Complex sentence: A big problem for the Communists was the increase of Chetnik influence, which was strongest in south-eastern Bosanska Krajina and the adjacent north-west central Bosnia, in the zones of responsibility of the 3rd and 4th Krajina Detachments.
Split into simpler sentences: A big problem for the Communists was the increase of Chetnik influence in that territory. This influence was strongest in south-eastern Bosanska Krajina and the adjacent north-west central Bosnia, in the zones of the 3rd and 4th Krajina Detachments.