Note on Venus and Anchises
some shepherd king to meet
Deep in the hollow of a shaded vale
To make his woes a long-enduring tale
Morris is probably referring to the story of Venus's seduction of the Trojan prince Anchises, a distant kinsman of King Priam, as related in the Homeric Hymn to Aphrodite. As he was keeping his herds on Mount Ida, the goddess came to him in the guise of a very attractive mortal woman, claiming to be under a divine command to wed him. After he had taken her to his bed, she revealed her divine identity (which he had initially guessed) and ordered him not to reveal their connection, to save her from embarrassment. (When he did so, Jupiter crippled him with a thunderbolt.) The offspring of their union was the hero Aeneas, the legendary progenitor of the Roman people. The later connection of Venus with a warrior from Thessaly (lines 1946--8) does not seem to be based on any amorous exploit of Venus/Aphrodite in ancient mythology; unless Morris has 'mortalised' her usual lover the god Mars/Ares.
Incidentally the 'strong-armed .... girls’ in lines 1715-17 are being likened to the Amazon women