Complex sentence: After protracted negotiation, the government of Venezuela granted all the demands of the United States 11 August; but, during the passage down the river, Perry was stricken with yellow fever and died before he returned to his flagship.
Split into simpler sentences: After protracted negotiation, the government of Venezuela granted all the demands of the United States on 11 August. However, during the passage down the river, Perry was stricken with yellow fever and died before he could return to his flagship.