Complex sentence: The day after the attack, Leighton's brain was operated on in an attempt to prevent a loss of speech; however, his brain was severely damaged resulting in the end of his pacification intentions to reunite the various groups opposing Pinochet, including the lefts.
Split into simpler sentences: The day after the attack, Leighton's brain was operated on in an attempt to prevent loss of speech; however, his brain was severely damaged. This event brought about the end of his pacification intentions to reunite the various groups opposing Pinochet, including the lefts.