Complex sentence: After this contact, two companies of Marines advanced on the hill 861, encountering heavy fire from entrenched NVA positions, constant mortar barrages on potential landing zones prevented evacuation of wounded and fog cut off most air support.
Split into simpler sentences: After this contact, two companies of Marines advanced on Hill 861, encountering heavy fire from entrenched NVA positions. Constant mortar barrages on potential landing zones prevented evacuation of wounded and fog cut off most air support.