Complex sentence: Located 20km from Son La, the fortified firebase had an airstrip that could accomodate the Dakota airplanes, had "rings" of outposts with a complicated trench system enforced with barbed wires and defended by 11 battalions (15,000 men) and 6 artillery batteries.
Split into simpler sentences: Located 20km from Son La, the fortified fire-base had an airstrip that could accommodate the Dakota airplanes, had "rings" of outposts with a complicated trench system enforced with barbed wires. Na-San had a defense force of 11 battalions (15,000 men) and 6 artillery batteries.