Complex sentence: Sweeps through the two valleys in late 1965 by the 1st Cavalry failed to find many Viet Cong who were believed to have fled the valleys, but to have returned after the 1st Cavalry withdrew to its base.
Split into simpler sentences: Sweeps through the two valleys in late 1965 by the 1st Cavalry failed to find large numbers of Viet Cong. They were believed to have fled the valleys, but to have returned after the 1st Cavalry withdrew to its base.