The True History (1.17)

As soon as the standards were raised and the asses on both sides (their trumpeters) had brayed, the engagement commenced. The Sunite left at once broke without awaiting the onset of the Horse-vultures, and we pursued, slaying them. On the other hand, their right had the better of our left, the Skygnats pressing on right up to our infantry. When these joined in, however, they turned and fled, chiefly owing to the moral effect of our success on the other flank. The rout became decisive, great numbers were taken and slain, and blood flowed in great quantities on to the clouds, staining them as red as we see them at sunset; much of it also dropped earthwards, and suggested to me that it was possibly some ancient event of the same kind which persuaded Homer that Zeus had rained blood at the death of Sarpedon.