Complex sentence: He acted aggressively in dealing with the Filipinos under the impression that their resistance would collapse quickly; even after this proved false, he continued to insist that the insurgency had been defeated, and that the remaining casualties were caused by "isolated bands of outlaws".
Split into simpler sentences: He acted aggressively in dealing with the Filipinos under the impression that their resistance would collapse quickly. Even after this proved false, he continued to insist that the insurgency had been defeated, and that the remaining casualties were caused by "isolated bands of outlaws".