Complex sentence: The Marines and helicopter crews never received the good intelligence available about the island's defenders; they went in expecting 18 to 40 lightly armed militia but instead found a reinforced battalion of elite Khmer Rouge naval infantry.
Split into simpler sentences: The Marines and helicopter crews never received good intelligence available about the island's defenders. They went in expecting 18 to 40 lightly armed militiamen but instead found a reinforced battalion of elite Khmer Rouge naval infantry.