Complex sentence: The author, a troop sergeant of the 15th Light Dragoons, looked upon Long as a peerless commander with virtually all the superiorities of organisation or training that he claimed for his regiment over the rest of the British cavalry ascribed to Long's initiatives when in command.
Split into simpler sentences: The author, a troop sergeant of the 15th Light Dragoons, looked upon Long as a peerless commander. He ascribed virtually all of the superiorities of organisation or training that he claimed for his regiment, over the rest of the British cavalry, to Long's initiatives when in command.