Complex sentence: Combined with the fact that the battle at Oriskany had cost so many Indian lives, this greatly upset the Indians, as they had been told that the white men, who had thus far fought relatively little, would do most of the fighting.
Split into simpler sentences: Combined with the fact that the battle at Oriskany had cost so many Indian lives, this greatly upset the Indians. They had been told that the white men, who had thus far fought relatively little, would do most of the fighting.