How does Auden contrast the past with the present in the "Shield of Achilles"?

The myth of Achilles is occupied by Auden in order to demonstrate the contrast between the audacious past and unheroic present. The folktale of his passed collocates of the present. in the poem, the theme portrays a place founded on principles and ethics. But when expression such as "an unitelligible multitude" and "column by column in a cloud of dust", are used, they quickly lose their value. The poem is an allegory of contemporary times.
Auden is disgusted by the modern world. He feels it is too regulated and unethical. the Achillean world uses the world as his microphone to comment on the lack of growth and development of the modern world.
The shield reflects the hollowness and pointlessness of a life.
The sky like "lead" on the sheild reflects the cold, frigid, and passionate-less human behavior
The modern life is based on logic and reasoning. It is described as lack of feeling.

In “The Shield of Achilles,” Auden sets up stanzas that alternate between the past and the present as a way of taking a critical look at the modern world. The first, fourth, and seventh stanzas all begin:

She looked over his shoulder.

When Auden uses "she" and "his," he is referring to Thetis and her son Achilles, the great Greek hero/warrior. In each of these instances, Thetis is looking into the past. "Over his shoulder," she expects to see the beauty and culture of Ancient Greece engraved or reflected on Achilles's shield. For example, in the first stanza, Auden writes:

She looked over his shoulder For vines and olive trees,Marble well-governed cities And ships upon untamed seas,

However, instead of the beautiful landscape and the interesting architecture, she has a prophetic vision of the present:
But there on the shining metal His hands had put instead An artificial wilderness And a sky like lead.
The two longer stanzas that follow show a darker image of the modern world, one that is barren with faceless masses of soldiers, barbed wire, and empty justifications for war and violence. Auden makes an allusion to the crucifixion of Christ as a symbolic look to the darkness of the world that follows the Ancient Greeks.
In the final stanza, Auden looks to the past again:

The thin-lipped armorer, Hephaestos, hobbled away,Thetis of the shining breasts Cried out in dismayAt what the god had wrought To please her son, the strongIron-hearted man-slaying Achilles Who would not live long.

Here, we see the heroes and gods of the past turning away and dying while Thetis cries out "at what the god had wrought." Here, Auden refers to the shield that Hephaestos has made for Achilles. This can also be seen as a way of looking toward the future from the past. The root of Greece's downfall was in the violence of Achilles all along, and it planted the seeds for the darker world of the present that was to come. This is why she always sees the future "on the shining metal" of his shield.
https://poets.org/poem/shield-achilles