Complex sentence: This third OZNA section was in force until the end of July, 1945, when the "military" and "civil" part of OZNA began to separate and finally split in March, 1946.
Split into simpler sentences: This third OZNA section was in force until the end of July, 1945. The "military" and "civil" part of OZNA began to separate in 1945 and finally split in March, 1946.