When Thetis and Peleus got married
Apollo stood up at the sumptuous wedding feast
and blessed the bridal pair
for the son who would come from their union.
'Sickness will never visit him,' he said,
'and his life will be a long one.'
This pleased Thetis immensely:
the words of Apollo, expert in prophecies,
seemed a guarantee of security for her child.
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Apollo may be interpreted as not really unfaithful. It was fate that caused it. Or inexperience. How would a kid fare in battle? Judging from the time table for these events, Achilles could not have been more than ten years old when he was made a warrior. From the abduction of Helen up to the time the Greeks mustered enough ships to sail for Troy, it was only a few years, for sure, any husband would not tarry in taking back his wife. The Trojan war lasted ten years, and so Achilles must have made his 10th Birthday in the battlefields. And as to gods, they change sides as often as they wish. You made a Homeric attempt here, and well.